Most people think they love someone when they find someone they have feelings for, Yes those feelings can develop into love but usually people have it mixed up with fascination or Lust or even both.
What causes them to break up? Life issues, they realized they weren't meant to be together, fights, conflicts, immaturity, lack of chemistry,etc, the list goes on and on. It just depends on what happened.
I think it's because most couples don't make it past the honeymoon phase of the relationship. Once things start to get real and they get to know each other they pick up on things that you dislike about the other person. If you can accept them for their habits you don't like then the relationship survives. If you can't accept it then it ends.