Brosemer starts with a list of hard things: stress, frustration, confusion, pain, suffering, death. The items grow heavier as the list goes on, and the trailing dots after "Death" leave a heavy silence. Then the speaker turns these troubles into something ordinary: specks of dust that gather water until they fall as rain.
The key idea is about quantity. Each speck is tiny, but there are "so many" of them that a light summer shower, the kind we hope for, always becomes a thunderstorm. The speaker seems to be saying that life rarely sends one problem at a time. Troubles pile up and arrive together.
The ending is quiet and almost personal. "Maybe now you will understand / Why I hate the rain?" The question suggests the speaker is tired, a little defensive, and wants to be understood. The mood is heavy and weary, but also honest. Rain here is really a stand-in for feeling overwhelmed.