James Joyce said that "literature is the eternal affirmation of the spirit of man". It is quite common place now, if you think about it, but it's most definitely true.
Poetry is many things: technically speaking it is music, numbers, regularity, beautiful imagery, and I think that aspect of it shouldn't be underestimated; but I believe poetry might be looked at as an unprecedented, original perspective on the world. Every poet looks around him/herself and translates what he/she sees into verse, but not just for his/her own benefit. Rather than the expression of the self, I think it is the expression of what is universal to everyone, to human kind.
Keats once wrote that it was his duty as a poet to give voice to those who couldn't express themselves...maybe that's just what poetry is: a voice for the voiceless.
Sorry...it's quite an inconstistent explanation...I have thrown all sorts of hypotheses in there...hope some of it made sense
