Rain vs Pain

What is it exactly about the rain that causes so much pain for people with with Fybromyalgia, arthritis, depression, and other illness? Lack of sun and cold is easy, but rain? Water is a gift of the gods, we are mostly composed of water, we need to drink water to survive. The planet thrives and grown and reproduces because  of it. So why does it wreak havoc on us so? I guess that is the question of day.