Exactly. Burnout can happen because you have a cynical malaise about your mission. But it can also happen because you are so passionate about your mission that where a normal person would understand reasonable boundaries between work and life, you can't resist the temptation to push right past them. (Which violates the principle of taking care of yourself first, so that you can take care of others.)

Sorry to bring up religion in polite conversation, but an interesting theory I've heard about church pastors is that sometimes they have extramarital affairs (partly) for an unusual reason. They feel called to do their type of work, but it's all-consuming, so they end up feeling conflicted. Part of them wants to quit, but another part cannot justify it. So instead of quitting, they do something which might or might not get them fired. It's a moral failing, but less of a moral failing than abandoning your mission. (Ultimately, it's just a way to not take responsibility for your choices, but that's human beings for you.)