“I can’t understand why…” is a lazy way out. Most people who “can’t understand” why X thinks Y, or why A does B, can’t understand because they haven’t tried to understand. If you say “I can’t understand why…” then, have you asked? Have you read the literature? Questioned the doers and the believers? I think you would be surprised at how much you can understand. I think that the books of both Mother Theresa and Hitler would be equally understandable, though we admire the one and abhor the other.
And that brings me to the next thing. “I can’t understand why…” is not just sloth. It is also a peculiar brand of racism, and a particularly cowardly brand at that. Like all racism, it divides the population into humans and not-humans based on some arbitrary standard. And then, with the words “I can’t understand why…” we try and set ourselves apart from the other race, whatever that race may be. I can’t understand because we are fundamentally different. There is no commonality between us.
I can’t understand the hatred of Hitler because if Hitler was human, like me, I would have to confront the fact that I, too, am capable of unspeakable atrocities. I can’t understand the self-sacrifice of Mother Theresa because if Mother Theresa was human, like me, I would have to confront the fact that I, too, am capable of unspeakable love, but I have failed to show it.