Leech in a Jar

When everyone is differently minded in a group of people who all think the same. Typically a depressed person in a group of abnormally happy people.

Person 1 - "Look at Timmy. He's so pessimistic while everyone else is running around, having fun."
Person 2 - "Yeah, you could say he's a like a leech in a jar. He knows he'll just die eventually, what's the point of it all."
Person 1 - "Wtf man."