It just happens. People are changing all the time. Like for example, you're probably still not friends with the people you knew from primary school, right?
This is because as you grow older, you change, and the people around you change, you want different things, need different things. That won't fit in the lives of others.
Maybe your girlfriend thought that she was to young to tie herself down to a long term relationship. Or maybe she knows that it won't work out in the future.
Also, your love for others changes too, you may love them more, maybe less. And slowing they stop loving each other, and you couldn't be in a relationship if you didn't love someone right?