Zach was your best friend, he was a relatively normal child who didn't go out of his way to make people feel nice, or make them feel bad. You were the opposite, you were a super happy go lucky person, who made everyone happy, that's why he decided he want to play a prank.

He invited you to play truth or dare, You accepted and you were halfway through the game when you said truth and he asked as a joke

Zach: "Are you depressed?"

Seeing you freeze and your eyes widen, seeing the disbelief and guilt in your eyes made his heart sink.. He didn't think you were.. actually..