He genuinely enjoyed his NEWT classes, which were very small and contained only students who were actually good at the subject, and most of whom enjoyed it. That meant that he didn't need to be angry and terrifying all the time to get his students to focus on their work, and could actually focus on talking about Potions. NEWT classes were when he typically started discussing theory and how to alter potions recipes - which was the most interesting part of the subject - because any sooner and he'd likely have serious injuries every other day. So no, he didn't hate being a teacher, some parts of it were definitely great.