Nikon D7000 flash doesn't always go off?

I have a Nikon D7000 and was wondering why my cameras flash doesn't always go off? It seems to only go off when the camera sees it fit, but when the flash won't go off the pictures just don't come out as well as they do when it does go off. Does anyone know why it does that and if it is possible to program the camera to have the flash ALWAYS go off instead of just sometimes?

Yes. Use P mode and open the flash with the flash button. Auto mode will only pop the flash if the camera thinks it is too dark or there's too much contrast. Consider getting a speedlight if you use the flash a lot.