When bad weather or a mean aunt or some other force takes over and you still don't change, you're unaffected. If your party was unaffected by the rain and Aunt Emma's no-fun attitude, then everyone probably had a good time.
Unaffected has other meanings, too. It can mean natural and genuine, or unemotional. If someone says you're unaffected they could mean you're genuine; you don't put on an act for people. Saying you were unaffected by your favorite team's loss means you weren't upset by it.