http://www.happiness-project.com/happiness_project/2008/02/happiness-isa-t.html

One important source of happiness is intellectual gratification. Satisfying your curiosity, mastering a new subject, acquiring a new skill – these bring intense happiness. Also, intellectual gratification doesn’t necessarily involve other people. It’s clear that a critical – perhaps THE critical – element of happiness is strong bonds with other people, but I, for one, also enjoy retreats into solitude.