Im not sure about my relationship and im really not sure why?? Help please :p?

I've been with my current girlfriend for about, 4 and a half months now, and im starting to have second thoughts and i really dont know why, shes really nice, she would do anything for me, shes quite pretty, my friends have also said that they think she is good looking, my parents love her and hers love me, im 17 and she is 15, soon to be 16, why do i feel like this??...