The entirety of The Sacrifice is this as you see the how the characters from the first game have months later all formed an incredibly deep friendship.
The first time you finish the last campaign of any given game, especially if you played through the campaigns in order. This is the moment when you can truthfully say that Left 4 Dead is a modern-day Odyssey, not just a game about zombies.
In the Passing, when Ellis falls head over heels for Zoey. Love is not dead, even if everyone else is!