It really is a thankless fucking job. Every kid is some flavor of a piece of shit, some benign, some really damn awful. You bust your ass to maybe make some of 'em a few percent less likely to remain a piece of shit when they become adults. And then you get to watch most of that work be undone by other adults. And the coworkers who phone it in get the same amount of recognition as the ones who really care because kids are too repressed emotionally to thank their teachers genuinely and fellow adults are too busy to take the time.
And no, the smiles of children is not sustainable enough a high to get you through that. You build up your tolerance to it after a few months at the very latest. Just give these people enough salary and oversight for them to care enough about their work consistently so the schools cease churning out emotionally retarded children, please.