Lyric discussion by dancehallhips 

Cover art for Debate Exposes Doubt lyrics by Death Cab for Cutie

he's talking about observing people's actions, more specifically verbal actions such as "i'm sorry" or "i love you" and how they can lose it's meaning with repetition.

i believe the last bit is how if you think enough, life starts to make sense, but if you think too much, you start wondering if things are worth it when they really are, hence "debate exposes doubt". The end also obviously points out how he is a much better person when he is with someone else.

The song as a whole talks about relationships and how couples achieve for perfection and how obvious it is that everyone is trying for this perfection.

I'd like to see some interpretation of middle part of the song. it's hard to crack.