It is his love for his wife Padme that causes him this change.
A Jedi Shall not know anger, nor hatred, nor love.
Anakin knows all these things that he should not.
He is actually the victim of the Star Wars saga, we learn this from the prequels. It is rather sad.
He is 'the chosen one', the one who will bring the force back into balance.
Anakin Skywalker: Or be with the people I love?
Padme Amidala: Are you allowed to love? I thought that was forbidden for a jedi?
Anakin Skywalker: Attatchment is forbidden. Possesion is forbidden. Compassion, which I would define as unconditional love, essential to a jedi's life. So you might say, that, we are encouraged to love.
Emperor Palpatine: The force is strong with you, a powerful Sith you will become. Henceforth, you shall be known as Darth...Vader.
Obi-Wan: You were the Chosen One! It was said you would bring balance to the force, not leave it in darkness!
Padme Amidala: I'm not going to die in childbirth Ani, I promise you.
Anakin Skywalker: No I promise you!
Obi-Wan Kenobi: You were the Chosen One! It was said that you would destroy the Sith, not join them! Bring balance to the Force, not leave it in darkness!
Darth Vader: I HATE YOU!
Obi-Wan Kenobi: You were my brother, Anakin. I loved you!
Luke Skywalker: I've accepted the truth that you were once Anakin Skywalker, my father.
Darth Vader: That name no longer has any meaning for me!
Luke Skywalker: It is the name of your true self. You've only forgotten. I know there is good in you.
Despite his gruff demeanor, Watto treated the Skywalkers fairly. After meeting Qui-Gon Jinn, he dreamed of becoming a Jedi Knight. His chances were enhanced when Qui-Gon learned of his immaculate conception, believing he to be the Chosen One prophesied by the ancient Jedi lore.
After winning the Boonta Eve Classic podrace and being freed by Qui-Gon, he left Tatooine and trained by Obi-Wan Kenobi. His own lightsaber was constructed on Ilum, but he could only remember obtaining the red blade from a vision of Darth Maul. Eventually, he grew into a brash young pilot who joined the Old Republic as a fighter pilot, gaining recognition during his education at the Academy and then during the Clone Wars.
He was still a brash young man, and the barriers of patience and hard work seemed to stifle his growing proficiency in the Force. It was about this time that Senator Palpatine also noticed the young Anakin. Sensing the Force within him, Palpatine also sensed his desire to learn quickly. After the death of Shmi Skywalker Lars at the hands of a clan of Tusken Raiders, he became so enraged that he killed the entire clan, inclding the females and the children. His descent into the Dark Side began. During the Battle of Geonosis, he lost his right arm in a lightsaber duel with Count Dooku. The arm was replaced with a cybernetic prosthesis, but took away some part of Anakin's humanity.
Seizing the opportunity, Palpatine introduced Anakin to the wonders of the Dark Side of The Force, and Anakin was hooked. He realized that, with the power of the Force behind him, he would be able to provide for himself and his pregnant wife. There would be nothing to hinder him, and his security seemed assured. He became Palpatine's student, and the fall from the Light Side to the Dark Side happened so quickly that Kenobi was powerless to stop it.
Kenobi pleaded with Anakin to return to the Light Side, but Anakin would have no part of it. After a heated argument, Anakin and Kenobi confronted each other in a vicious lightsaber duel. They were evenly matched physically, but Obi-Wan's knowledge of the Force was much greater than Anakin's, and Kenobi was able to maintain his strength. A coincidence led to their battle nearing a pit of molten lava, and Anakin misplaced his footing. He fell into the pit, and was seared alive. Obi-Wan, seeing that he had a single chance to save his friend and return him to the Light Side, pulled the burning Anakin from the pit. He returned to the Old Republic, where a tram of cybernetic surgeons worked furiously to save him.
The newly-promoted Emperor Palpatine spared no expense in trying to save Anakin. This was noted by Kenobi, who realized that Anakin would see Palpatine as a savior. Kenobi then fled with Anakin's wife, hiding her and her unborn twins on remote worlds, out of the Emperor's watchful sight. Palpatine, upon Anakin's recovery, re-introduced Anakin to the Dark Side of the Force. This eventually led to his acceptance of the Sith training and lore, and sometime during this training Anakin Skywalker ceased to exist. He became Darth Vader, and rose to become a Dark Lord of the Sith.
As such, he also became Palpatine's minion, and became the Emperor's tool. Sometime after the Battle of Yavin, Palpatine was able to discover that Anakin had had a son. He used this knowledge to help secure his hold on the former Anakin Skywalker, as well as a way to help turn Luke to the Dark Side. The Light Side of him seemed to try and return to the surface, and Vader was faced with a crisis. During the Battle of Endor, Luke managed to get the Light Side of Anakin to re-assert itself, and as
the Emperor was trying to kill Luke, Anakin rose up and threw the Emperor down the Death Star's power shaft. The life-support systems that kept Darth Vader alive, however, were seriously damaged by the Emperor's Force lightning, and failed. He died soon after Luke removed the facemask to allow Anakin to see his son with his own eyes.
After Qui-Gon used his force powers to cheat on a dice roll and then influenced the outcome of the race (how else could a kid who's never actually finished the race before WIN against the greatest racers in the galaxy?), Anakin was freed from his slavery].
He was then taken from his mother (who had given birth to him without having slept with any man... YEEEEAAH RIIIIIGHT) to be trained as a Jedi]. But apparently, ten was too old to be trained, so he was then taken into the middle of a war on some garden planet that everyone made a big stink about. He accidentally destroyed a Trade Federation ship which none of the elite pilots could get close to doing. Lucky bastard (no really, he was lucky, and he was the the illegitimate offspring of unmarried parents).
He was then taken as the Padawan of Obi-Wan Kenobi (who only trained him because it was the last wish of his dying master Qui-Gon Jinn).
It is important to note in this part of the story that one of Anakin's abilities to age ten years in the same time it takes Obi-Wan to grow a beard.
Both of them do just that while everyone else stays exactly the same.
Anakin resumes his attempted romance] with the beautiful Padme, but is turned down as he was when he was ten. But she still had his plastic trinket he made for her! There was still hope!
So Anakin took her from the well-guarded facilities of the Coruscant capital to a primitive country-side where she was virtually unprotected. He did this for her protection of course.
While in the countryside, Anakin's feelings grew for her as her clothes began to become more revealing and tighter with each scene.
Anakin proceeded to say tons of corny crap and talk about dictators, all of which somehow swayed Padme in her feelings, so she began to love him.
Then he kissed her, and got pissed at her about it.
She refused to engage in sexual relations with him and he began whining.
Both then proceed to get captured on a planet across the galaxy, where they profess their undying love to one another right before their execution.
Padme gets cut perfectly across the middle by a rat monster in a way that reveals her bellybutton and abs just right, which completes her skin-tight outfit's appeal, coming close to rivaling Leia's bikini in Return of the Jedi] (the right monster should become a Hollywood] fashion designer); and Anakin loses an arm.
Both are married].
Then Anakin turned to the dark side over one nightmare in which Padme dies. He did this in an attempt to save Padme, which is kind of ironic considering he later strangles her to near death (but don't worry, in a last minute revision, George Lucas] decided that she should die of... lack of will to live? So Anakin's not to blame... apparently).
Anakin got fried after failing to beat his former master (which is kind of funny considering Anakin beat Count Dooku, who Obi-Wan had previously not coming close to even touching, in a minute as well as being the acclaimed 'most powerful Jedi').
I suppose it's cause Obi-Wan had the higher ground.
Anakin then became Darth Vader], and went around blowing up planets.
I feel as if everything is going wrong!
My loyalty to the emperor and the council leave me feeling torn and conflicted!
Padme, kiss me, or I will feel neglected!"
Geez, man, will this guy ever quit whining?
Anakin Skywalker is Darth Vader. Darth Vader is Luke's daddy.
Oh, and that's a spoiler.