The single drive underlying all human activity, top-to-bottom, front-to-back, is the creation and development of more little healthy baby humans. You can actually apply that logic to all life forms, so it's not that big a stretch.
In the "Gain Pleasure / Avoid Pain" motivational paradigm (which are absolutely the sole reasons we do ANYTHING), babies are Pleasure on a cellular level. That great sense of deep Joy you feel is our genetic reason for being.
And congratulations on your baby. ;-)