@tsusagi
Maybe I'm wrong, but I remember that way back when he was introduced, the teacher was feeling... disappointed to be a teacher, so I kinda feel it has to do something about it as well. That, as time went on (and motivated by the Home Economics teacher that moved away), he changed his opinion about that and he is still trying to do it as he didn't want to give it up so easily.
Or it could be related to the whole guitar hobby as well, that, compared to Hi Rang, he did give up the guitar and couldn't simply pick it up again as he still can do it, as a means of motivation. I mean, Hi Rang gave up the guitar for other personal reasons and he was giving up covering up excuses to justify why, instead of just feeling honest about it.