If a fast-food worker can make $15, many people who are in jobs that currently pay less, or not much higher, will have an incentive to leave their jobs for one that pays better. As a result, employers who underpay their workers will feel pressure to increase their wages. This process will repeat itself, each time at a higher wage level, until everyone's wages will go up.

Seriously, this argument is incredibly selfish. It always depresses me that people can be so self-centered. If we're so near-sighted that we only care about ourselves and don't want anyone else to do better, we will never achieve much as a society. I hope most people are smart and thoughtful enough to be able to see the trees and the forest.