Someone has been abandoned, and the speaker can't stop asking why. The first stanza is a string of "what could I have..." questions: what could I have done, said, or even thought to make you leave? The speaker doesn't blame the other person at first. Instead, they search themselves for a fault, even imagining that a private thought might have been enough to drive someone away. That kind of self-questioning is a familiar part of heartbreak.
The second stanza changes the pattern. The speaker asks why they feel pain, sadness, and loneliness, then answers each time with a blunt "because you." The replies are short and plain, and the confusion of the first half gives way to a kind of hard clarity. The ending slows right down: "From me" gets its own line, and "Forever" stands alone, heavy and final. The mood is raw and quiet, with no anger shouted, only hurt that has settled in and doesn't expect to lift.