Time for a little trip back in time to when George was still a schoolboy. One of the other kids there was Dobbin, a clumsy, dorky kid, who was mocked mercilessly because his father was a merchant and had to work for a living rather than being independently wealthy. This all has to do with being or not being a gentleman – a really big deal for Thackeray. Check out the "Character Clues" section for some info on the whole "what being a gentleman really means" thing. Go ahead. We'll wait for you.
Another kid at the school was Cuff, who was the popular, bullying jock type.
Everyone who was there will always remember the day Dobbin saw Cuff about to beat up George. It's not really clear how old everyone is at this point. We'll hazard a guess that George was 9ish, Dobbin 11ish, and Cuff 14ish. Point being that Cuff was way bigger than George.
Dobbin intervened, and he and Cuff planned to fight after school.
The fight was very much one-sided, with a half-crazed Dobbin "licking" Cuff (that's some awesome period slang for you all).
After this, Dobbin's reputation went way up, and the other kids started being so nice to him that he actually began doing really well in school. Turns out he wasn't stupid after all, but just too miserable to study or otherwise function.
The other result of the fight? Dobbin and George became best friends. Or rather, George condescended to let Dobbin be really attached to him.
Since all this happened, however, Dobbin's dad has become an alderman and has gotten knighted, meaning he is now Sir Dobbin, and is thus a much higher social class than when Dobbin was at school. Dobbin is now Captain Dobbin and is an officer in George's regiment.
The reason for all this history? To explain why George invites Dobbin to Vauxhall as kind of a fifth wheel.
Dobbin comes over to the Sedley house, sees Amelia, and instantly falls in love with her. The five young people hang out, having a good time, until it's time to go. Jos has a few strong drinks for the road, or maybe to give himself some liquid courage to ask Becky to marry him.