Why does anyone fall in love? Presumably the human did something or posseses some quality that impressed them. Perhaps there is something in human nature that the demons don't have, but this one in particular has learned to respect. If they're a wizard, that could be something like curiosity...
Depends what you call 'modern', maybe. Pre-industrial Europeans tended to wake up for a few hours in the middle of the night, and there are sources from the 17th century that discuss medics and clergymen using the time to study by candlelight.