Keeping the design as good as it can be

Agile developers [...] never say "We'll go back and fix that later." They never let the rot begin.

[...]

The attitude that agile developers have toward the design of the software is the same attitude that surgeons have toward sterile procedure. Sterile procedure is what makes surgery possible. Without it, the risk of infection would be far too high to tolerate. Agile developers feel the same way about their designs. The risk of letting even the tiniest bit of rot begin is too high to tolerate.

[...]

Professionalism dictates that we, as software developers, cannot tolerate code rot.

Robert C. Martin, "What is Agile Design?", in Agile Software Development:
Principles, Patterns, and Practices, 94.