The speaker looks around and sees a whole generation performing happiness. "We are all just sleep-deprived teenagers" makes the feeling sound universal: everyone is tired, living a life they never picked, and getting through it as best they can. The small details are telling. People laugh at jokes not because they're funny but "to make people believe we are alive," as if laughter were a costume. Even the music is described in a quietly grim way. It "doesn't want to make us die," which sets the bar for comfort very low. Music here isn't joy, just something that keeps despair at bay.
The last lines hold the sharpest twist. Everyone is "convinced we're happy inside," yet everyone also has "a reason to cry." The word "convinced" suggests self-persuasion rather than real contentment. The mood is weary, honest, and a little numb, but there's also a thread of solidarity. The speaker says "we," not "I," so the sadness is shared, even if nobody admits it.