Sometimes women just want to argue, and they'll find any reason at all; or just make one up out of thin air.
They're not content just being happy all the time. The emotional part of their brain gets bored. They enjoy riding the emotional rollercoaster. You'll notice that the guys who get the most girls don't just make the girl happy: they make the girls happy, sad, angry, scared, releived, etc. over and over again.