That's obvious. I'm not saying that should be the only reason, but it can play a part. You feel some personal connection to him or her, because of something else besides the same old politcal talk.
If you really don't see a big difference between the parties, or would like to feel represented by race or belief that does have merit.
If you are a Mormon, and you've never seen a Mormon president. He probably has many of the same values as you, or you'd like
to see what a Mormon would do in office. That's understandable. Why not? If Harry Reid was running for President you might vote Democrat. Voting the man, and his personal beliefs over his party.
You would likely see very, very few LDS republicans vote for Reid for president. Of the ones that would, most all would be heavily left-leaning to begin with.