He said that in all of his bravado, he never thought that his planet wouldn't be the one. He assumed that he would find the livable planet, people would come to him, and he'd help restart civilization.

Instead he landed on a dead planet and realized he would never see anyone ever again. So instead of killing himself, he sent out messages saying that his planet was great and people should come check it out.

Where Nolan screwed things up is that, instead of having Damon's character hear that people wanted to go to a third planet, a trip that would use all of their fuel and leave them no shot at returning to Earth, he has Matt Damon actually telling Matty Mc to not focus about getting home to his family and to stay focused on the mission.

It's a horribly botched section of the film because you can see it coming from a mile away yet it still leaves you somewhat confused and the climax is a limp fist fight.