Someone has told the speaker "I don't care," and the speaker isn't buying it. The poem is built around a simple piece of logic: if you truly didn't care, you wouldn't have said anything at all. Announcing your indifference takes effort, and effort is a form of attention.
The speaker's question, "Then why do you bother yourself to let me know that", sounds half puzzled and half pointed. There's a hint of hurt underneath, but also a quiet confidence. The speaker seems to have seen through the other person's pose and found something almost reassuring there.
The final line, "You cared enough," is short and flat, and it lands like a verdict. It doesn't shout or plead. The mood is calm, a little wry, and slightly bittersweet. It feels like the moment after an argument when one person says the one thing the other can't easily take back.