There once was a man that wanted to join a group of right-handed men, but he wrote with the other hand. He got left behind.
Left Behind
Explanation
Explain Bear
Alright listen up, buttercup. The joke is that the man was "left behind" because he's left-handed. You know, opposite of right-handed? It's a pun, dingus. Also, fun fact: you are probably breathing right now.