Seinfeld + Harold and Maude + Peanuts = Sayonara, Zetsubou-Sensei.
That's pretty much the only way I can describe the way this series feels. The protagonist doesn't just see the glass as half-empty, he's convinced someone else has spat in it out of spite. To make matters worse, he teaches a class full of losers, delinquents, and plain old freaks, including a girl who sees things as positively as he does negatively - so much that she's convinced people who hang themselves are just "trying to make themselves taller."
Much of the humor comes from Koji Kumeta riffing on Japanese culture. This could have been a disaster for American readers, but thankfully the translators at Del Rey pack the end of each volume with pages full of translation notes and cultural trivia, (something they do for many other series,) making Sayonara, Zetsubou-Sensei almost as informative as it is hilarious.