One reason that challenge brings happiness is that it allows you to expand your self-definition. You become larger. Suddenly you can do yoga or make homemade beer or speak a decent amount of Spanish. Research shows that the more elements make up your identity, the less threatening it is when any one element is threatened.
As I read this, I thought about the relatively easy transition (at least emotionally) I've had with going from full-on unschooling mom to doing my own thing and how comfortable I've been as my kids have gone off on their own.
Because I've continually added elements to who I am, it's been easy for me to let go of the bits and pieces that have made me who I am in the past. Of course, one of the biggest bits (or piece?) was unschooling mom. That was my main gig for over 12 years - my top priority all those years.
But as I've (sometimes selfishly) engaged in new bits and pieces, I've been able to fill in the cracks and holes that come from big changes.