Does infatuation ever die down?

You should read Helen Fisher's book "Why We Love" or any of her other books. She has done many studies on people who are in relationship or infatuated with someone, and even going through a breakup. It is really interesting.

Basically Helen says by being infatuated with someone you are releasing chemicals in your body that would mimic the feeling you get while on herion (I think it was heroin, I know it was some kind of drug) So that is probably what you feel when you start getting interested in someone. Since you said that you have never had a girlfriend you are so used to having that feeling that once it starts dying down you are ready to move on. Helen talks about the feeling of attachment also but it doesn't sound like you have that with any of the girls, but are you close with your mom?

In my experience I wouldn't say love get sombre but you do learn everything about the person. But it is exciting because we are always there for each other to motivate to reach our goals, and sometime we teach each other things that we never tried before.

Don't be afraid to get your heart broken, I don't know how old you are but as the saying goes "it is better to have loved than to have never loved before"