Another thing I realized after posting (and I think this didn't come through in my post).
One silly one-line truism that has crept into my life philosophy (from Gretchen Rubin's "The Happiness Project") is "Happiness doesn't always feel happy." I kind of feel like there are two kinds...one derived from pleasure (momentary/temporary) and one derived from satisfaction. Getting satisfaction isn't always pleasant, but knowing you did, or are doing, something important does tend to give you long-term happiness.
Graduate school is not a pleasant job. I have days when I am horribly bored, or confused and frustrated, or anxious, or start getting depressed. But I believe my field and my work are important, and the days when things work give me the satisfaction to get through the other days. And while I can't say I would never take a job just for the paycheck (we all need to eat!), I think I can say that I would try to get out of it as soon as possible if I didn't enjoy the day-to-day OR believe in its greater mission.