in every life a little rain must fall

In every life a little rain must fall is another way to say that in every persons life there will be some trouble, even if it is minor and may turn out to be beneficial. It is another way of saying that nobodys life is perfect even if it seems so. That for each person, something that seems bad on the outside will happen and in the end it turns out to be a good thing, such as in the case of rain helping things grow but can also be a burden to some.