If you give your girl the world and more why do they still seem to cheat?

i was in a four year relationship and i gave my girl the world and more whatever she asked for i gave her, and still in the end she played me talked to others and told them she was single, i gave her another chance and she still played me for some reason my dum *** is still here, if it was me and i had all these second, third chances i would straighten up, PLEASE someone tell me why girls cheat....then say men are dogs....