So much togetherness...plus:
Leonard, Sheldon, Howard, Raj, and Amy are all colleagues.
Penny and Bernie work together.
Leonard, Howard and Sheldon share a patent.
Stuart’s Comic Book Store is each guy's favorite watering hole.
Sheldon and Amy live across the hall from Leonard and Penny.
Raj is Godfather to Howard and Bernie’s kids.
Stuart lives with (and sits for) Howard and Bernie.
Sheldon’s relationship with Leonard’s mom is more congenial than Leonard’s.
In the real world, all this personal and professional blending would have made people tired of looking-at-each-other; long time.