Why do babys pull hair?

Our remote evolutionary ancestors were probably arboreal and modern arboreal animals usually have young with an instinct to grip their mothers fur and hang on tight as the parent uses its hands and feet to climb and jump. It is possible that the instinct in babies to grip and hang on to hair is a leftover from this period in our evolution. Babies just love pulling anything ! Give him/her some other things to pull at and they'll likely play around with those and hopefully leave their hair alone.