To dwell in a home is to live in it. To dwell on something — usually something bad, like a failed romance or terrible service in a restaurant — is to think or speak about it at great length.
Dwell has been part of the English language for over 1,000 years. The definition of the word has evolved considerably since its first recorded use in the 9th century when Old English dwellan meant "lead astray" or "hinder." If you constantly dwell on the bad things in life, it could hinder you from seeing the good or lead you astray to the negative. If you keep complaining, your friends might tell you to stop dwelling on it or, if they're really annoyed, they'll say, "Get over it!"