For several reasons:
a) because my mother was depressed, and on an unhappy marriage but I was a happy child, well loved and looked after.
b) because it is one of those trite meaningless phrases trotted out to justify choices made by the for the benefit of the parent.
c) happiness in general has been given hugely inflated value and is a very individualised concept. What makes you happy can be in direct conflict with what is good for you, your family and society in general.
There are more reasons but I have to go and get the paper and some groceries for the family though it would make me happy to stay harping on about happiness on Mn!