The Big Bang Theory Episode 3.19 The Wheaton Recurrence

Leonard tells Penny he loves her but she replies with "thank you" which was not the response that Leonard was looking for. Leonard is hurt and Penny feels him pulling away and explains to him that she has said "I love you" too early in a relationship and the outcome wasn't good so she didn't want to rush into it and ruin what they have.

Sheldon, Penny and the guys meet up with Stuart's team at the bowling alley but Stuart has a surprise replacement for a regular teammate - Wil Wheaton. Sheldon thinks this is his moment to take his revenge but Penny and Leonard get into a fight while bowling and both storm off making Sheldon forfeit the game to Wil's delight. Sheldon facilitates Leonard and Penny's reconciliation solely for the reason that he needs both to play a rematch game with Wil Wheaton and his team. The game is pretty close and Wil hones in on Penny's insecurities about saying "I love you" to Leonard, essentially playing mind games, in order to throw off her game. His tactic works and Penny runs off yelling at Leonard and Sheldon must forfeit to Wil once again. But Sheldon vows he will get his revenge.