A grouch is never happy. Nothing good ever happens to him. Even his friends, he feels, never treat him right. A grouch is always griping and complaining about something. When something good does happen to him, he manages to find something wrong with it.
So what if his friend accidentally does something he doesn't like? "He did it on purpose! He's just trying to make me mad!" the grouch will decide. And soon enough, he'll tell someone about it... and he'll be speaking Loshon Hora. Someone who thinks others are always "out to get him" is sure to speak Loshon Hora.
This man is wearing "grouch glasses" - whichever way he looks and whatever he looks at he manages to see people and things which make him upset and angry.