Best answer I've ever seen (it's a old poem, but it still holds true):
Sorority Poem
A sorority is not entirely a flower, national conventions, monogrammed rings, worn out songs, by-laws, membership standards, or a gold pin. And it's not entirely an institution, a creed, a legacy, an obligation, or a way of life.
It is long eternities of black coffee and exam snacks, when you still can't remember the Renaissance paintings of the mossy tap root system.
It is feeling proud of the accomplishments of your sisters.
It is sitting on the back steps and listening with all your helplessness because she's so lost and she's so lonely, and it seems the whole world just fell into ugly little pieces.
It is studying for the big exam while a special friend types your ten-page research paper. And it is coming in late one night and closing the door to tell someone that had seen you through the hardest years of your life that you are happy now, and you are going to be married.
And a sorority is, I suppose, a kind of evaluation.
In these halls, perhaps you'll learn more about this grizzly, ungrateful circus we call life than if you'd lived it somewhere else.
You learn that a football player is sometimes just shoulder pads and skinny arms sometimes hide a great man.
You learn that the lecture halls are just watery echoes, and that they are silent rooms for your deepest rivers.
You learn that no matter where you came from, or who you took there, you've still got to find that one small acre that belongs to you, by yourself.
You learn that the world of today is made up of people you're not going to like, and you live with them anyway.
You learn to wait, because change is slow, and change isn't always right.
You learn that there is a lot to believe in, and a whole lot more to hope for.
You learn that love has never been easy, and it's a long time coming.
And if you're smart or very lucky, you learn that no matter how big or messy the world becomes, it is precious. And what is precious, and what is permanent, is always the same.
Also, check the "Right reasons for Going Greek" thread.