Here are a few reasons why a girl would suddenly lose interest in a guy:
1) We find out you're not sincere and your motives are just to get into our pants.
2) You are a jerk and hit on/date other girls while dating us and keeping it a secret, just be upfront please.
3) You lied.
4) We realize that there is no common interests and so there is no conversation.
5) You start getting really clingy, which scares us off.
6) You have too much baggage and are an emotional mess that we just can't deal with it.
7) You cheat.
8) You are way too into yourself, bragging and talking all about you and only you.
9) You never show us an affection and or that you care about us.
10) You make us cry more than we smile.
Those are a few examples of why a girl would lose interest in a guy, from my experiences anyway. Hope that helps :)