A flashback in the season three episode “The Staircase Implementation” reveals that the two characters met when Leonard applied to be Sheldon's roommate. That moment, we are told, was “seven years ago”, and the episode aired in 2010, meaning that Sheldon and Leonard presumably met in 2003.
It dictates what Leonard has to eat each day of the week, what TV shows he is allowed to watch, whether he has a girlfriend stay over, that he has to drive Sheldon everywhere he wishes to go, whenever he wishes. It even dictates when Leonard is allowed to use the bathroom.
Sheldon and Leonard live together out of convenience not for financial reasons. Sheldon has said in the past that he wouldn't have a roommate if he could afford the rent. Penny on the other hand who lives alone on a smaller wage, is seen always struggling to get by, living paycheck to paycheck.
A Roommate Agreement is a written contract between roommates that outlines their rights and obligations while living together. This agreement includes house rules, maintenance duties, restricted behaviors, and more. For a Roommate Agreement to be useful, everyone sharing the household must agree to it.
A flashback in the season three episode “The Staircase Implementation” reveals that the two characters met when Leonard applied to be Sheldon's roommate. That moment, we are told, was “seven years ago”, and the episode aired in 2010, meaning that Sheldon and Leonard presumably met in 2003.
My take is that at the time when he was interviewing for a roommate, he was just freshly starting out at his job so the rent was too expensive for him to afford alone, but as his work in the university gradually secured him a steady income years later, he earned more than enough to live alone, but he simply chose not ...
Sheldon met, and eventually became friends with, Leonard when Leonard became his roommate. Through his budding friendship with Leonard, Sheldon also became friends with Howard and Raj, who were already Leonard's friends.
Frank and Alicia are some friends of Sheldon, who are only mentioned in "The Fermentation Bifurcation". Sheldon mentions them as alternatives to his regular friends whom he figuratively likes to keep under his sink and have to maintain a little mystery with his regular friends.
The Big Theory initially centers on five characters: Sheldon Lee Cooper and Leonard Hofstadter, two physicists and roommates; Penny, their neighbor who is a waitress and aspiring actress; Sheldon and Leonard's friends and coworkers aerospace engineer Howard Joel Wolowitz and astrophysicist Raj Koothrappali.