[Retroman]

Louis is the geeky unappreciated type who may have been bullied at school.

Simon is the assertive, confident type who would have intimidated Louis, as well as make Louis want to be like him.

Louis is now older, and a little famous, so he's gained enough confidence to talk back.

So now he spends all his moments constantly having a go at Simon, and unleashing his years upon years of built up frustration on Simon in a desperate bid for revenge.

Louis bares a grudge against all the times Simon has talked down to him/made Louis look less intelligent/proved a point against Louis etc etc, and lives to say whatever he can to somehow bring Simon down to his level.

Seriously, just watch Louis over the entire series and notice his constant references to Simon and how he tries to belittle him in a petty like manner.

Then he'll also decide to insult other people in order to win Simons respect, when that happens Louis and Simon usually laugh together and Louis looks really happy he's gained Simons positive attention.

It doesn't take a psychologist to notice these things, he's incredibly see through.