I'd add 3: most of us really like this stuff. Sometimes I get pinged in the evening for some issue. I have the freedom to say I'll check it out in the morning, or on Monday. Sometimes I do, but often I'll look at it. There's definitely a little bit of 2 in there, but mostly it's because checking it out is fun, and solving it is rewarding.

I try not to overdo it. You can still burn out from that sort of thing even if you like it. But as long as it's not excessive, I enjoy the occasional "Sorry it's late, but are you around to take a look at this?"