[GeorgeKangas]

Why is no one looking at the chance-giver, and considering his motive? One would reasonably assume he'd like to minimize the payout -- so he'd offer you the chance to switch only if you had chosen the richer envelope. The description explicitly allows this possibility:

"Having chosen... you are given the chance to switch"

Nowhere does it say, you'll be offered the switch no matter which envelope you choose.

Same idea applies, to the very similar Monty Hall problem.