The speaker keeps circling the same few thoughts: something, or someone, has been left outside in the rain, and it has happened before. The poem never says who "they" are, and that vagueness lets the guilt feel larger. They could be people, pets, or even parts of the speaker's own life that have been shut out.
The repetition does most of the work. "Again" appears over and over, and the questions loop without answers, which feels like a sleepless mind at night. The rain keeps dripping through the cracks of the window, so even the speaker's shelter isn't fully safe. The title hints at this: the barrier between inside and outside is already broken.
The mood is tired, anxious and self-reproachful. The speaker calls themselves "cruel" and then "forgiving," as if unable to decide whether the fault is neglect or too much patience. The last line, with "little left till dawn," adds a faint sense that change is coming, but the speaker still hasn't moved.