Congrats to all the darkersides.
A new born baby weighs almost nothing (7,11 not insubstantial) but after a few weeks you suddenly feel a gain in substance, this is similar timing to your mind slowly realising what you have done and how you have become responsible for another human being's wellbeing. And you feel good about this. You get surprises like how much you actually like the fact they fall asleep on your chest etc.
As the time rolls by you get to a point where you start to feel you might possibly be getting a bit bored and then the really clever stuff starts. They start smiling at you or making the sound da da or mamma that almost every human on the planet interprets in the same way. Or they look right at you and light up or they start bum shuffling or sticking their hand in your mouth. Some very intelligent design has gone into the concept that engages the caregiver for a lifetime.
Enjoy.