The joy of debugging other people's programs
There is a peculiar joy that is probably more common among sysadmins than programmers: the pleasure of having successfully debugged someone else's program.
Triumphing over a bug in your own program is already a pretty good feeling (assuming that it is a tricky bug, not something stupidly obvious that leaves you wanting to smack your head). Doing it to someone else's program gives you not just the reward of having squashed the bug but also the accomplishment of having sliced through enough of the program to find the problem and the place to fix it; you've triumphed over both the bug and the code.
(Also, in the sysadmin world generally you are fixing a bug not because it's there or been reported to you by someone else, but because you ran into it and it's causing you pain; fixing it gives you a direct reward.)
This joy, and the prospect of it, can be peculiarly addictive. In the stubborn pursuit of it (or simply the stubborn pursuit of irritating bugs) I've dived into all sorts of dark corners of various code bases at all hours of the day and night; sometimes successfully, sometimes unsuccessfully.
(In part I think it's addictive because understanding more and more about the code is in itself a reward. Even if you seem no closer to the bug, this increased understanding gives you that feedback you crave and it feels like progress.)
Some of the time this following has been to excess. It's an easy thing to get pulled into, especially if I feel that I'm making progress, but sometimes the right answer is to accept that I'm not going to find the bug any time soon and I need to move on somehow. This is not necessarily the right answer for a programmer, but it can be the necessary answer for a sysadmin. Still, it's not a joy that I like giving up.
(Also, I think that programmers are tacitly trained to not give up on bugs; it's part of the culture that you hunt them down sooner or later, even if they're obscure and hard to find. We pass around war stories of heroic debugging and impressively peculiar bugs, for example.)