In just three short lines, the speaker tells us something small and a little deflating: every can he finds has already been flattened. The poem doesn't say why he's looking, but the wording suggests someone out walking, perhaps collecting cans, and expecting at least one to be intact. Instead, each discovery is the same disappointment.
The feeling is quiet and a bit weary. There's no anger or big complaint, just a plain statement of what keeps happening. The word "all" makes it sound like a pattern, as if the speaker's luck never changes. The short lines and the abrupt ending, with "crushed" landing last, give the poem a flat, heavy finish, like the sound of the thing itself. It's also easy to read it more broadly: the crushed cans can stand in for anything that arrives already used up or broken before you reach it. The poem leaves that idea open, and its modest tone is what makes it stick.