Why was teaching so pleasant for Kit?
An interesting question, and one with several answers.
We're told flat out that she looks forward to teaching because she'll be earning money, and this will help the family accept her, or at least, not wish she were a boy. She also thinks about how much more pleasant teaching will be than the hard physical labor of cleaning and farming, so it is pleasant in part because it is easier.
Working with words (teaching reading) allows Kit to remember/glimpse the pleasure she had learning with her grandfather. She flat out likes reading and stories, and finds that she really enjoys retelling them.
Though we're not told this explicitly, I would also say Kit enjoys teaching because she dislikes being under the watchful and commanding eye of others, and for a time she is relatively free in the classroom.
After ten days of working at the dame school as a teacher, Kit realized that those ten days were the pleasantest she had experienced since arriving in Wethersfield. I can think of two reasons for this. First, Kit now had a job to do that she felt she was good at. It gave her a sense of purpose that she did not feel when she was working on the farm with her cousins. Second, at the school, she and the children had quickly connected with each other. She felt accepted in a way that she didn’t feel with any of the other people in the town. It was the first time she felt like she fit in.