Often, if I don't enjoy the course you're teaching, I'll project some of that distaste onto you, the professor, and my memories of you will not be fond.
It's just that simple.
My statistics professor is throwing this out of whack, though, because she is so sincerely nice.
I have no idea what I'm doing in this class. Absolutely no clue. It's a best-guess scenario every time I open my book. Yet, I find my professor to be endearing. I think I amused by the way our classes go: we'll be using a football team's weight and height statistics for a problem, and as she works through the problem she'll begin to talk about how beat up her brother was when he played high school football. And then you realize you've got "Football players--lots of bruising" written in your notes.
This should bother me, but it doesn't. What I like must be that she is logical and friendly: sort of like an aunt or your mom's cousin, and she always remembers to get you a birthday gift, but it's something like wool socks or a set of encyclopedias. And when you open them you vaguely know that in the middle of winter or while writing a research paper you'll be really, really thankful for her practicality, but you're not sure there could be a less exciting birthday gift. And I think it's the thought that counts, even if right now the thought is that I should be learning math.