The value of money.

It's really interesting here how hard it is to get change for bills higher than 20 reais. If I ever give a 50 reais bill, I usually am met with resentment and I'm afraid that evil eyes are being cast on me by the cashier. I am too afraid to give 100 reais bills-- and it's a problem, because the banks tend to only give out 50 or 100 reais bills.

Is there a reasoning behind this? If it is so hard to find change, why do the banks continue to give us such large bills?