How may compulsive programmers be distinguished from a merely dedicated, hard-working professional programmer? First, by the fact that the ordinary professional programmer addresses himself to the problem to be solved, whereas the compulsive programmer sees the problem mainly as an opportunity to interact with the computer.Ö. The professional regards programming as a means to an end, not as an end in itself. His satisfaction comes from having solved a substantive problem, not having bent a computer to his will.
    The compulsive programmer is generally a superb technician, one who knows every detail of the computer he works on, its peripheral equipment, the computerís operating system, etcÖ. His position is rather like that of a bank employee who doesnít do much for the bank, but is kept on because only he knows the combination of the safeÖ.
    Unlike the professional, the compulsive programmer cannot attend to other tasks, not even to tasks closely related to his program, during periods when he is not actually operating a computer. He can barely tolerate being away from the machine. But when he is nevertheless forced by circumstances to be separated from it, at least he has his computer printouts with him. He studies them, he talks about them to anyone who will listenóthough, of course, no one else can understand them. Indeed, while in the grip of his compulsion he can talk of nothing but his program. But the only time he is, so to say, happy, is when he is at the computer console. (Weizenbaum, 117-18)

If there's one thing I've learned from working with computer programmers, it's that they constantly defie expectations. Just when you thing you've got them figured out they throw you a curveball. Maybe they break down and tell you about the puppy they lost when they were six, or maybe someone who hasn't seemed to obsess over computer science suddenly spends 74 hours straight in the lab, or something equally unexpected. The point is, by defying expectations they also defy being placed into Weizenbaum's neat little buckets. Still, he does come up with some good psychological explanations, although I believe they apply to all coders, not just the "compulsive" ones:
 

    The psychological situation the compulsive programmer find himself in while [fixing a bug in his code] is strongly determined by two apparently opposing facts: first, he knows that he can make the computer do anything he wants it to do; and second, the computer constantly displays undeniable evidence of his failures to him. It reproaches him. There is no escaping this bind. The engineer can resign himself to the truth that there are some things he doesnít know. But the programmer moves in a world entirely of his own making. The computer challenges his power, not his knowledge.
    Indeed, the compulsive programmerís excitement rises to a fevered pitch when he is on the trail of a most recalcitrant error, when everything ought to work but the computer nevertheless reproaches him by misbehaving in a number of mysterious, apparently unrelated ways. It is then that the system the programmer has created gives every evidence of having taken on a life of its own, and certainly, of having slipped from his control. This too is the point where the idea that the computer can be "made to do anything" becomes most relevant and most soundly based in reality. For, under such circumstances, the misbehaving artifact is, in fact, the programmerís own creation. Its very misbehavior can, as we have already said, be the consequence only of what he has done. And what he has done he can presumably come to understand, to undo, and to redo to better serve his purpose. Accordingly, his mood and his activity becomes increasingly frantic when he believes he has finally discovered the source of the trouble. Should his time at the console be nearly up at that moment, he will take enormous risks with his program, making substantial changes, one after another, in minutes or even seconds without so much as recording what he is doing, always pleading for another minute. He can, under such circumstances, rapidly and virtually irretrievably destroy weeks and weeks of his own work. Should he, however, find a deeply embedded error, one that actually does account for much of the programís misbehavior, his joy is unbounded. It is a thrill to see a hitherto moribund program come back to life; there is no other way to say it. When some deep error has been found and repaired, many different portions of the program, which until then had given nothing but incomprehensible outputs, suddenly behave smoothly and deliver precisely the intended results. There is reason for the diagnostician to be pleased and, if the error was really deep inside the system, even proud.
    But the compulsive programmerís pride and elation are very brief. His success consists of having shown the computer who its master is. And having demonstrated that he can make it do this much, he immediately sets out to make it do even more. (Weizenbaum, 119-20)