He comes from some webcomic. There's a panel where he says feels good man after being asked why he takes his pants off when he shits, and it became a reaction image way back. Then it evolved into feels bad man and eventually just sad frog. Because the character from the comic is named Pepe, people call him that.
>japanese mangaka stole our frog meme and uploaded on social networks
He browse /a/?
Slaughtered in the most inhumane way
He realized frogposting is the meaning to life.