There are alot of levels to this joke. It makes the point that if God really did play favorites, we would probably know. But in truth, rain falls on the just and unjust alike, and it is up to us to see the rain as a blessing or a curse. There is a quote from The Prophet that poetically makes this point: "Surely he who is worthy to receive his days and his nights is worthy of all else from you. And he who has deserved to drink from the ocean of life deserves to fill his cup from your little stream."
Who are we to determine who is to receive the blessings we give out? God does not act so. In this world she does not go around smiting evil-doers and blessing do-gooders with aught but the fruits of their own labor. So how can we judge others as worthy or unworthy of our gifts?
Another aspect is whether we see what happens to us as a blessing or a curse. Our culture gives the impression that most people see hardship as curse. But I have come to see it as, in many ways, a blessing. Having the requirement to sacrifice something is an opportunity to build and improve oneself. It is my sense that too much prosperity will disconnect people from the truly important things in life and cloud one's life and mind with useless things.
If it is raining on you, it is your choice to enjoy it or lament it. Personally it seems much better for me and my community if I learn to enjoy the hardships of life.