I actually think that the formulation "most important aspect" deserves to be questioned or qualified. Too often, a useful idea becomes "reified," turned into an absolute standard by which everything else should be measured.
But human life is not reducible to one "master idea." Humans are (among other things) complexes of competing/cooperating systems, desires and perceptions. We don't just want one thing, and we don't just have one of seeing things, or doing things. So in my opinion, it's better to speak of "most important aspects"--plural.
So, some of the most important aspects human life, in my opinion:
--Connection. We need one another. The myth of the castaway--Robinson Crusoe, or the Tom Hanks character in "Castaway"--is powerful because one of the most challenging possible experiences for most humans is such complete and utter isolation--even when we can satisfy our material needs by our selves.
--Meaningful work. We are much happier if our lives contribute something to the Universe, or to our community or family. The frustration of a job that may satisfy our material needs, but appears to the worker to be ultimately meaningless and pointless is also a persistent theme, and one most of us can understand.
The work need not be paid; parenting is very often the most meaningful job of all. Even relatively casual volunteerism can also be quite satisfying. But there is in many of us a real desire to 'make a difference.'
--Transcendence. We also seem to want to experience reality in ways that feel deeper or more powerful than our mundane experience. There is no society and no culture without some form of myth, some form of religion, or some forms of art and music. Experiences and ideas around this need seem to vary a lot among individuals and societies, but without experiences of transcendence, people become restless, dissatisfied and prone to erratic behavior.
Why we need this is not clear to me. Is it because we are "spiritual creatures," whatever that phrase means? Is it because we can't ever come to complete answers--precisely because we are complexes of competing/cooperating systems, for which no one answer can ever be completely relevant? Our drive toward mystery is itself mysterious, and seems likely to remain so.