Gladness is the right response to existence, not a reward for having earned something. After the locust years, after the mind has eaten its own field bare with its worry and its calculation, rain comes anyway. It was never withheld as punishment and it is not now given as wages. It falls because falling is its nature.
The early rain and the latter rain point to something the mind cannot arrange: what is needed arrives in its own time, in just measure, and the one who trusts this stops living in the anxious future. Celebration is that trust made visible. A man who dances in the first drops is no longer negotiating with life; he is drinking it.