I think empathy is about acknowledging how others feel, and not why. As for not understanding why others react the way they do, I myself sometimes react certain ways when I am in a certain mental state, but later laugh at myself why I reacted that way when I am in another mental state. In other words, a person himself/herself doesn't understand himself/herself at different times. Your mental state is likely not the same as others at any given time, and the reactions won't be the same for the same things. We see the world through different lenses.