The theory goes like this. Rigby is dead and Mordecai is in a coma. One night they have a guy's night out and get drunk. Skips and Muscle Man fall asleep while Benson is at his apartment and doesn't want to be bothered. Mordecai wants to go to Margret's house and chill. He takes the cart and drives drunk. A big truck hits them and they are not able to survive the crash. Keep in mind, this is based on the episode, "First Day" where Mordo and Rigby are so happy to be working at the park and that night, they celebrate. Skip everything in the episode after the scene where Mordecai and Rigby are seen "getting drunk off soda". Skips then wakes up in panic when he realizes that they are not at the house. Skips finds the cart a few blocks away from Margret's house and finds Mordecai unconscious. Unable to wake Rigby he takes them both to the hospital. He notifies everyone to get there as soon as possible. The doctor pronounces Rigby dead. Mordecai is in a coma and although he cannot speak is awake. Distraught by the death of his only best friend, Mordecai makes "Regular Show" a compilation of his dreams. He creates a place where he can always be with his best friend.
This explains a lot about the show, and how it can be post- apocalyptic. It also explains why they can do so many dangerous things without getting hurt. In some of the other episodes, Rigby falls in a coma too. Like in the episode "Eggselent" where Rigby gets an allergic reaction to a popular menu item at a diner. Or the time when Skips kills Rigby by mistake while trying to win an arm wrestling challenge. Mordecai probably did this because he didn't want to blame himself for what happened and couldn't take the fault of accidently killing his best friend. Did you make it all the way through? Good! Now that you read it, how does it change the way you feel about Regular Show? Like I said before, I made this theory, so if there are flaws in it, please say, but don't be a jerk about it.
Thanks again guys, and I'll catch you in the next one!
MZ. CARTOON!