Busy people rarely get bored. We won't go to Mars for vacations, we'll go to establish a colony. Which means there are a myriad tasks that have to be accomplished on any given day. They'll have to make sure their equipment works in stellar condition otherwise they'll die, build new things, explore, grow plants and taste their food, run experiments, fix things, take notes of everything and report back to Earth, and the list goes on. And once they get there we'll keep sending them new stuff, and probably other people too, quite frequently. Mars is indeed a barren place but imagine being a geologist, it's like Christmas, you have a new world all to yourself to explore and learn new shit. That could keep you busy for a lifetime.

Sure, in the long term they might get depressed assuming a trip back to Earth would be impossible. And that's why we'll send mosty scientists. We want people who are dedicated and passionate about exploration and learning.