Everyone has that one t-shirt that says something: "Writing is my superpower" or "Pirates hate ninjas" or "Really, Officer, I was only looking for an ammo can." (That last one was a geocaching reference.) Everyone has something to say, and a t-shirt is one way to say it.
This is actually an interesting study. Why not start collecting these phrases? You're prob'ly thinking, "Of all possible things to collect, why t-shirt phrases?" Because they mean something, at least to the person who's wearing the t-shirt. And writers want to convey meaning, don't they? Of all possible messages, why did that person choose this one? Of all possible people, who did you least expect to wear their message?
If I were to gather just the phrases I'd seen among my family members, you'd see Bible verses, "No forks allowed," "Due to unfortunate circumstances, I am awake," patriotic phrases, and a dozen other funny, heartwarming, and usually meaningful thoughts. Try it.