Actually, it's in everyone's best interests to share in a relatively equal manner (not exactly...just relatively). If you visit a place where there is a vast gulf between the incredibly rich and the desperately poor...and you contrast it with a place where everyone or almost everyone is doing reasonably well...you'll see exactly what I mean about that.

It is not in the true interests of anyone for there to be poor people. It makes life nasty and dangerous for the whole society in a great variety of ways. Misery creates further misery.

A society is best off if everyone is at a decent level, materially speaking...and if everyone feels reasonably secure about their future.

The same goes for a family, and a society is a very large extended family.

A stupid and shortsighted rich person thinks he's better if almost all the rest are poor. A wiser rich person realizes he's far better off if everyone is doing well.