(I was so very tempted to write an answer and not upvote, but annoyingly someone's already given it an upvote so my asinine joke would've been wasted ;) )
I feel like I upvote a lot of questions regardless of whether I write an answer, but I don't upvote every question I answer. I tend to upvote questions where:
a). I think it's likely to be useful to other people, or
b). I find it interesting, or
c). I get the impression that the asker has put considerable effort in, either attempting to solve the problem themselves, or spending the time to narrow down/explain what their issue actually is, or
d). I just get the nebulous feeling that the asker deserves an upvote.
I use completely different criteria for deciding whether to answer a question:
a). I know the answer, or
b). I don't know the answer, but it looks interesting/fun and I think I can figure it out, or
c). I just want to get it off the Unanswered Questions list.
The two don't always line up.