I found it due to feeling transference for ex-marriage counselor. I didn't find him attractive at first, but it grew out of the transference. (And if I look at a photo of him now, which I haven't done in a while, I also don't think of him as attractive.)

Meanwhile, my current T, Dr. T, I found to be attractive immediately. To the point that my first thought when I met him was "Uh-oh, he's attractive in person, too. Am I making a terrible mistake?" (I did tell Dr. T this at one point, and he was amused. And said he hopes "Am I making a terrible mistake?" isn't most people's first thought upon meeting him.)

So I think it's common to be attracted to a T, whether it's due to transference or someone you would just find attractive if, say, you say them on the street. Or from getting to know them, like someone becoming more attractive to you over time (or less attractive, for that matter).