Four short lines carry a whole relationship's worth of disappointment. The speaker is talking to someone close, and the complaint is simple: what this person says and what they actually do don't line up. There's no shouting or long list of grievances here, just a plain statement of the problem.
The real feeling comes in the last two lines. "What happened to us? / I wonder" sounds quieter than anger. The speaker seems more puzzled and sad than furious, looking back at a closeness that has slipped away and trying to work out where it went. The word "wonder" leaves the question hanging, as if no answer is coming.
The mood is hushed, tired, and a little lonely. The brevity helps: the poem simply stops, the way someone might trail off when they've said the same thing to themselves many times. The speaker is left with the gap between promises and behavior, and with the memory of something better.