The main problem is that we are raised to always give, give, give and it's not allowed to spend time with yourself; if you do that, you are selfish. Look at 2 year old children, they are punished if they like to play alone with their toy, they are forced to share and if they rather play alone they are called weird.
The result is that everyone is afraid to be left out and most are not able to spend time alone and next they visit doctors as an adult telling them to be more selfish and if not they will never feel happy.
If it wouldn't be so sad it would be a joke.
You are right if you want to feel happy you should learn to spend qualitime time with yourself.

A happy Monday!