why there are guys that suddenly lose interest with a girl they love? :'(

If a guy/man truly loves you then they will never lose interest in you. When your your young more often then not there is that strong attraction that one person has for another which is often mistaken for love. When it turns out that it wasn't love at all you are left with a situation something like this. When a person loves you they love every part that is you, not just some things about you like lets say your looks etc. etc. It seems to me that you have to take another look at this relationship & ask yourself some real hard questions. More then likely you already have the answers that you need, but most people are too afraid to look within themselves that deeply because of what they'll find there.