if you can't fathom why they do what they do, or why they believe what they believe, that's your failing not theirs.
Interestingly though, by accepting this symmetry between you and your enemy, you potentially thereby break it. If you can understand why they believe what they believe, but they don't understand why you believe what you do, then you can justifiably consider yourself in a superior epistemic position.
Maybe they also think they understand you. You can't get intelligence from simple asymmetry.
(Since there didn't seem to be one for this month, and I just ran across a nice quote.)
A monthly thread for posting any interesting rationality-related quotes you've seen recently on the Internet, or had stored in your quotesfile for ages.