This proverb literally means that every time it rains, it pours ("but" in this context means something like "without" or "unless.") Obviously this isn't literally true. What the saying illustrates is the idea that, when troubles come, they often seem to come all at once; or at any rate, no sooner has one problem been dealt with than another appears. As Shakespeare put it in Hamlet, "When sorrows come/ They come not single spies, but in battalions." Or to put it more simply, there is the old country saying "Troubles (or deaths) come in threes."
This probably isn't just superstition. Of course, if you are feeling overwhelmed by one difficulty, then even if something minor comes up it may feel like a major issue (or you may not have time to deal with it.) But it can also be literally true; if you are struggling to deal with one thing, then usually trivial difficulties can actually (not apparently) become big ones. To take an everyday example; in the middle of a domestic crisis you are more likely to have a car accident, say, or forget to pay an important bill, than you normally would. Thus you soon find that you are dealing with several problems at once.
This probably isn't just superstition. Of course, if you are feeling overwhelmed by one difficulty, then even if something minor comes up it may feel like a major issue (or you may not have time to deal with it.) But it can also be literally true; if you are struggling to deal with one thing, then usually trivial difficulties can actually (not apparently) become big ones. To take an everyday example; in the middle of a domestic crisis you are more likely to have a car accident, say, or forget to pay an important bill, than you normally would. Thus you soon find that you are dealing with several problems at once.
