Homecoming
Sheldon didn't like to leave his room. It meant one thing: people. Work people, home people. He wasn't the world's biggest fan of people. If he could, he would live on the planet of Solaria where all contact was done through holographic telepresence. He wouldn't even need to be in the same room with people.
However, when Sheldon Cooper walked through his door and saw Leonard gone and Penny trying to crack his password on his laptop, he realized something. Something that changed his mind about living on Solaria. Solaria didn't have Penny. And Penny didn't count as People.