Why dont i love my girlfriend?

Shes the love of my life (mind were only 18), shes gorgeous, shes funny, shes extremely sexy, shes still a virgin, and there's nothing i couldn't say or do to make her feel any different about me. but the only problem is we dated the year b4 and we were both in our first relationship which went perfect until i broke her heart, then we had a falling out and after a while we started talking and eventually went to prom together.. anyways we got back together and things picked back up perfectly, now she tells me all the time how much she loves me and how im so much to her & i feel the same way its just now whenever things start getting intense between us, i don't get that same flame or spark that i used too but i know i still love her more than any other girl in the whole world.... WHATS GOIN ON??