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.