We're professionals. We want to do a good job. We feel bad if someone finds fault with our code. If QA finds a bug, it makes us sad. If a user of our system encounters an error, we may cry. We should strive to deliver quality, error-free code and a fully functional, feature-rich application.

We're also lazy, but it's the good kind of lazy. We don't want to have to run the entire application just to validate that a simple function returns the proper value.