All dog breeds have a purpose. Historically, the purpose of the Shih Tzu was to be a companion and that's just what they want to be. They simply desire to be with you. So don't expect them to hunt, guard, or retrieve; that's not their style.
Affection is their dominant characteristic, and your lap is their favorite destination. They're happiest when they're with their family, giving and receiving attention.
That said, the Shih Tzu is not a total couch potato. They're alert and lively and may bark at newcomers to their home. Don't worry, though; they'll make friends with your guests the minute they walk inside.