I have a theory about happiness.
Happiness is a closed circle, not an endless container you can fill and fill. Its limits are our humanity and finite ability to invest energies.
If you invest more energy into one slice, the others must shrink. It takes the space allotted to the other slices, and happiness in those arenas decreases.
When you have a child, that child is an extra slice that you are blessed with, but that must be inserted into the pie…somewhere. It will affect the other slices, but with energy and vigor, you may not even notice.