But what it really boils down to is that friendship affords us a more dimensional way of looking at ourselves and at the world, thus enhancing our understanding of the meaning of life. Once again, Pigliucci takes us back to Aristotle:

Aristotle’s opinion was that friends hold a mirror up to each other; through that mirror they can see each other in ways that would not otherwise be accessible to them, and it is this (reciprocal) mirroring that helps them improve themselves as persons. Friends, then, share a similar concept of eudaimonia [Greek for “having a good demon,” often translated as “happiness”] and help each other achieve it. So it is not just that friends are instrumentally good because they enrich our lives, but that they are an integral part of what it means to live the good life, according to Aristotle and other ancient Greek philosophers (like Epicurus). Of course, another reason to value the idea of friendship is its social dimension. In the words of philosopher Elizabeth Telfer, friendship provides “a degree and kind of consideration for others’ welfare which cannot exist outside

I thought Eudaimonia was better translated as "human flourishing":

http://en.wikipedia.org/wiki/Eudaimonia

One is more shorthand than the other.

Okay.

But it seems circular that "friends lead to a good life" since a good life is defined as one with friends!

Circular or symbiotic?

Could be both, actually.

http://pandawhale.com/post/23218/ill-be-you-and-you-be-me-a-vintage-ode-to-friendship-and-imagination-illustrated-by-sendak-brain-pickings

Friendship is one of the more common Brain Pickings themes, I've found.