Clouds stand in for the things that set our mood, and the poem treats them almost like rulers. When the sky is bright, everyone is happy. When it turns grey, people cry and the storm seems to hunt for someone to hit. The weather changes, and so do the people under it, with hardly any choice in the matter.
The last stanza shifts from description to warning. The speaker says "Look," as if pointing something out we've missed, then states that the cloud controls you, whether it is bright or grey. The sharpest line is "And you just let them." The speaker isn't only sad about the storm; they seem frustrated that people hand over their feelings so easily. "Forever" lands like a door closing. The mood is gloomy and a little accusing, with a plain, blunt voice. Judging by the imagery, the cloud could stand for circumstances, other people, or any outside force we allow to decide how we feel.