This is an excellent article about understanding technical debt:

It is a fact that, over time, all development teams get slowed down by the existing codebase. But why? Is it because maintenance is inevitable? Or because we could do something better in the first place? Or both?

Luca argues that technical debt is introduced as a by-product of disagreement, which itself is a by-product of two phenomena: wrong design, and rapid evolution. Thoughtful stuff. Well worth your time.