James Chastek has an excellent post on why metaphysics (in the philosophical sense, not in the metaphysical bookstore sense) is so hard. About metaphysical proofs for the existence of God, he says:

Theist proofs that attempt to be scientific might have some value- I have little ability to judge them. They seem far too human to me. They occur in the comfortable human world of the most reasonable explanation. The metaphysical proofs are like staring at the sun or trying to hold your breath and explore deep underwater. Things are there, and there is occasional clarity; but much is blurry, you can’t see it for long, and you have to keep going down again and again to get a clear view of even relatively shallow things.

Read the whole thing; it’s short, and (unlike metaphysics) both clear and straightforward.