In 'love' terms, Obsession is when you are trying to control someone that you think you are in love with. You get them or try to get them, to do what you want them to do. If you break up with them you get all angry and want to destroy their lives and hurt them (inside). This is true obsession.
Thinking about someone loads is NOT Obsession.
A guy gets finished by a girl and gets all his friends to send hate mail to her friends and her.