Our purpose to expand into space is to propogate and protect Gaia
Looking at life on Earth on a cosmic scale, we ar part of a global ecosystem called Gaia, which "knows" staying on one planet forever is not viable to its survival, whether it be from resource depletion, impacts from space objects, or the eventual death of our star.
Perhaps it is all just our doing, but it does give the human race one important meaning and purpose, which it is always seeking. And this one is tangible.
http://en.wikipedia.org/wiki/Gaia_hypothesis
