A common way people lack empathy is failing to understand that a difference between two people can last a lifetime. Imagine you have a friend with trait Q. You may think you know what that is, but they have always had it, and you never had. For you Q goes away when you forget.
For them it never does. And because it is always there, it affects them in all kinds of ways. Imagine Q combined with every other thing that appears in life. Prima facie, any of those could interact. You forget Q exists; they get surprised by interactions between Q and life.
Now you tell them they don't make sense because you fail to remember Q, or when you remember Q, you fail to appreciate that even they are constantly surprised by interactions between Q and other things. You know enough to know you don't know, but then you say "You make no sense."
Or, worse, you distort what you hear and observe, and you leap to inferring all negative motives (prejudice in one of its many guises), giving a convincing appearance of intentional deafness. Rather than learn something or simply understand where another person is coming from, you decide that it's your job to judge what's relevant and important and understandable.
If this is your approach, don't be surprised if that person concludes that you're being stupid and insensitive.
