In some ways, it is a retelling of the love between Odin and Loki, in Norse mythology, or the awkward alliance between Romeo and Mercutio. From the best small town to the most corrupt city, one will find such friends across America -- pairs in which one is straight laced and honest and the other is constantly in trouble and needing help.
The key to this friendship surviving is, of course, the fact that in is own perverted way Jack actually does give back to Miles. Just as Loki always gets Odin in trouble and then comes up with a grand scheme that profits them both, Jack finds a way to better Miles' life. In this story, Jack may almost destroy the relationship betweem Miles and Maya, but one ought not forget that without Jack's interference that Miles would never have found the courage to start dating Maya at all. The fact that they do become intimate is as much Jack's doing as the fact that their new found intimacy is challenged by his philandering. Jack has money and spirit (one thinks he may come through with his promise to help Miles self-publish, and he certainly will fix Mile's car), and his annoying friendship helps keep Miles stable.
Though Miles' enduring friendship with Jack can be seen as prototypical of the sort of relationships which endure between trouble-makers and their serious friends, one can also shift one's perspective and analyse the way that Jack continues to be Miles' friend. If one can overcome the fact that Jack is entirely abnormal with his adulterous ways, and imagine him as the normal one in the pair, one can see how the movie analyzes the friendship between a regular happy-go-lucky individual and a friend that has a constant state of depression stemming from his lack of success in life and marriage and an over-intellectualization of the world. Even as each of us may have friends that are troublemakers, it is also likely that we each have friends sunk in depression. Jack's case shows the difficulty of living with someone who refuses to help themselves, and also the way that such a friend can provide an opportunity to reassess one's own life. Miles keeps Jack a little more level-headed than he might be otherwise, and also gives a great object lesson as to the sort of life that Jack wants to avoid. Without Miles' interference, Jack may never have realized how important it was to actually commit to his marriage.
In conclusion, the characters are far from normative. They suffer from a sort of moral prolapse that appears to have been brought on by too much money, wine, and the unhealthy moral air that presides in California. They are too overdramatic, oversexed (in Jack's case), and over funded to be normal Americans. Despite this, they obviously have some basis in human reality, for the shape of their friendship is such that one can see parallels to it in everyday life no matter in what sector of…