I think the novel abundantly examines several philosophical concepts. For example, it raises the philosophical pondering of how we all lead our lives. If we are going to be victimized by the same tragedies in life, and we will, then why should we care or why should it matter if we lead them morally or not? We will all experience suffering and despair. It compares Gabriel and Troy, who are on the opposite sides of responsible living. Gabriel is always concerned with doing the right thing and being responsible, whereas Troy is always pushing the envelope and embracing a reckless attitude.
A second example, I think, is the inequality in love relationships. One partner loves more and deeper than the other which ultimately causes resentment and destruction. Therefore, should we avoid falling in love to save ourselves from the eventual catastrophe?