How does love change so fast

One minute he loves you the next he loves her how does that work if you think about it, it hurts on the inside and you just don\'t want to think about it but you know that one day in your life it was going to happen and people just wont let you be happy with who you want so they all of the sudden change or you get to thinking that you were just a brat and he quit loving before he even got a chance to start