What is the issue with Thoreau's effort or approach to teaching?

Thoreau discusses his search for the simple life, one that will allow him freedom for study and contemplation. He knows he needs to earn some amount of money to afford even the stripped-down life that he prefers. He sees no point in working full time in a profession he wouldn't enjoy to earn money to buy consumer goods he doesn't desire. He catalogues various ways he has tried to earn living, such as teaching and trade. He notes:

I have thoroughly tried school-keeping, and found that my expenses were in proportion, or rather out of proportion, to my income, for I was obliged to dress and train, not to say think and believe, accordingly, and I lost my time into the bargain. As I did not teach for the good of my fellow-men, but simply for a livelihood, this was a failure.

He says that his expenses for teaching were too high to make it profitable and that it took up his time, as well. He notes that he didn't do the teaching for a higher purpose or from a sense of calling—"for the good of my fellow-men"—but to earn money. He didn't find his purpose in it and didn't make much money from it: therefore, it was a failure as far as he was concerned.