That title is misleading. It suggests I'm about to offer advice, words of wisdom, a mantra, something. But I'm not. (Coincidentally, I first typed "Grade School"; I probably could offer a bit of advice about being happy in grade school, since that time was, all things considered, a reasonably happy period in my life.)
More appropriate titles and/or responses might be:
1. Can someone tell me how to be happy in grad school?
2. Is it possible to be happy in grad school?
3. How do you define "happy"?
4. When you say "happy in grad school," do you mean during the moments you are physically in the physical buildings in which your graduate education occurs? or while you are in the ontological state of "in grad school," as opposed to "in the workforce"?
5. And then of course there's the epistemological issue of "Would you even know if you were happy? How?"
(Impt. note: such terms as "ontological" and "epistemological" are used purely for mockery of grad school. I don't actually talk like this. Ever.)
Joking aside, I'd sure like to know if anyone's got a winner of an answer. I'm not, by any means, saying I'm unhappy. But I don't think I can say I've met one person (myself included) in a long time who, when asked about grad school, would respond, "Yeah, I'm happy." And it's not just people in English departments; history, French, and philosophy grad students aren't any different. For my part, I've had a headache for three days straight.