How to tell spouse that his bad mood is really getting me down?

When my SO is in a bad mood, he acts so grouchy that it invariably gets me down. I know that I shouldn't let my mood be affected by his, but it seems like more than half the time we talk, he is unhappy about something. He gets snappy, sulky or annoyed, or he "teases" me about any little thing that bugs him. How can I talk to him about this so he realizes that this is a real downer for me?