Being Polite

I walked with a friend to the newsstand the other night, and he bought a paper, thanking the owner politely. The owner, however, did not even acknowledge it. "A sullen fellow, isn't he?" I commented as we walked away. "Oh, he's that way every night," shrugged my friend. "Then why do you continue being so polite to him?" I asked. And my friend replied, "Why should I let him determine how I'm going to act?"