I think because death characterizes the end-point for all human existence, we tend to focus on the temporal instead of the eternal; immediate satisfaction over delayed gratification. Premarital sex, for instance, is something we seize onto because either we don't want to wait for sex within the context of marriage or because we want to grab all the pleasure we can out of life. Or internal sins, like lust, in which we look for temporal goods on this side of death/eternity, instead of looking for ultimate Goods that come to us from the other side.
Once our flesh and gone and we've passed through death, the only question that remains is whether what remains of the person (the "soul") is free from the guilt that our death-centric life brought on us or whether we carry that guilt to the other side.
This view is basically the view of Augustine, and Augustine's interpretation of Paul's flesh/spirit opposition, but put in the language of the non-Christian philosopher Martin Heidegger.
