We can't read our opponent's mind, so we can't know the answer to your question. I say don't assume anything. In fact, in my opinion, it's a rather nice gesture to offer a rematch after you win, risking the chance that you could have a terrible game to replace your excellent win. Again though, as above, don't assume anything.
As far as rematch etiquette goes: it doesn't exist. There is nothing inherently wrong with not offering a rematch, since when you play a game you agree to the terms of that game; you don't agree to some sort of three game match. Just like how one shouldn't complain about their opponent taking forever on their move if you made it a two hour game -- he's only doing the wise thing.
"rubbing it in"???
I always refuse a rematch regardless if I win or lose... Why? Because
1. The guy is so upset at losing he's more likely to use a chess engine to help him with the next game...
2. If I lost genuinely, then there's something about that guy's techninque that is just hostile towards my style of play...