Rockin' robin

For probably the past two weeks (one doesn't notice when things like this begin, only that they've been going on for quite a while), a robin who hangs out in the trees to the south of my house begins singing at first light (I hear him—I assume it's a him—at 5 a.m or earlier). And he continues to sing. All day. Really. All day long. He doesn't stop. It's now just after 9 p.m., and, probably because it's finally dark, he's quit for the day. But he'll start in again tomorrow, just as he has for days. And days.

I mostly like hearing him, especially when I first wake up; much nicer than waking to the sound of trash trucks. But quite honestly, he's beginning to make me twitchy. Why does he sing all day long? Is he trying to entice a mate? Warn other birds away? Obsessive-compulsive? Does he not eat? Or drink? And why isn't he hoarse by now? But I don't really want him to stop; I'd just like to know why he doesn't seem to be able to.