a-change
Explanation
The Joke
A man confronts his friend Dave, pointing out that Dave only ever wears ratty old t-shirts and jeans. Dave responds that he has been slowly transitioning away from old clothes, replacing them with new ones over the course of their relationship. When the friend expresses shock ("WHAT?!"), Dave explains that it wasn't subtle: he has been putting on fresh cologne and "George's Sure Shine" product. The friend is appalled that it "wasn't subtle" at all. Dave then declares, "I'm a diamond in a rhinestone-covered dumpster" and that he "has been rendered" -- meaning he sees himself as a treasure wrapped in a rough exterior.
The comic plays on the idea of a person very slowly changing their appearance and being hurt that nobody noticed, while simultaneously the changes were apparently so minimal or poorly executed that they genuinely were imperceptible. Dave thinks he has been making a dramatic transformation; to everyone else, he still looks exactly the same.
The Humor
The humor lies in the gap between Dave's self-perception and reality. He believes he has been undergoing a noticeable, even dramatic personal makeover -- getting subtly better-dressed over time -- and is offended that his friend hasn't noticed. But the friend's genuine shock suggests the "transformation" was invisible to the outside world. The final declaration that he is "a diamond in a rhinestone-covered dumpster" is funny because it is simultaneously self-aggrandizing and self-deprecating: he admits to looking like a dumpster while insisting there is hidden value within. It satirizes people who make tiny, imperceptible efforts at self-improvement and then feel underappreciated when nobody notices.