The poem opens and closes with the same line about how crappy life can be and how the poetic speaker doesn't know how to explain it.
Then it goes on to compare life's disappointments to a series of religious images, which gives off the idea that the speaker is a little bit mad at God for letting bad things happen.
Finally the poem winds up with an image of a man crying over all the pain that he's lived through, and the repetition of "I don't know."