Another framework for thinking about this comes from some of Jonathan Haidt's research in The Happiness Hypothesis: most people have a residual equilibrium of happiness.

One example from his book: if someone gets in an accident and becomes paralyzed, they'll suffer great sadness, but that first sip of orange juice through a straw through, man, what a joy.

So essentially humans will adapt to find sources of happiness as circumstances change.

Re parenting: it's not like you suddenly discover a hidden passion for changing diapers, rather, your body will adapt to find a way to derive happiness. And the interesting thing is, it will be totally genuine!