Usually for either people who don't like replying "I love you (too)" in public (or at all), or people just trying to be clever. Has the effect of making a girl want to punch you (as a jerk ) and want to make out with you (as a bad boy ) at the same time.
Taken from " Star Wars Episode V: The Empire Strikes Back", where Leia reveals to Han that she loves him, and Han replies, "I know."
Her friend: "What did he say?"
GF: "He ended up pulling a Han Solo"
HF: "NO. WAY."
GF: "Yah. So I punched him on his shoulder."
HF: "And then?"
GF: "We just made out the rest of the movie."