Diana knows what it means to be human - she knows it better than Clark, who only sees the good in most people, or Bruce, who only sees the bad.
I know you know Bruce better than that. He puts up a front like he only sees the bad, but it's a front and everyone knows it. IIRC even Frank Miller couldn't convince himself that Batman would give up on Harvey Dent, to take an extreme example off the top of my head.