My main motivation to teach is seeing progress in my students. That's why I like the students that are the most gratifying to teach. So I guess one of the biggest factors is someone who practices regularly. However, not just blind practice but students who practice actively and think about what I told them to do during lessons.
Also, I like the students who you can tell are very interested in the music (for example, listen to the music at home, acquire some outside knowledge about the music like other compositions by the same composer, etc). This makes it very interesting for me to be around them just on the level of a music lover.
Lastly, I appreciate students that are honest with me about their goals and intentions when they first begin lessons with me, so that I could see if we would be a good fit for each other and I could know what I can focus on with the particular student. This saves me the trouble of dealing with students who consider lessons to be a burden for one reason or another.
Now some reasons for disliking students:
Frequent cancellations
Arguing during lessons
Not touching a piano all week until showing up to the lesson
Not working on the things that I tell them to work on
Expecting a refund after missing a lesson and a number of other ways that one can try to lower the amount they have to pay the next month
Telling me a reason why they're missing a lesson (being in bed sick) that they themselves don't remember and a few months later mentioning that they were out of town in vacation for a week (why do you have to lie?!)