How to understand other persons.
How I did it: I think the main thing to realize is that everyone is more alike than different, and that everyone experiences pain. Some experience it short-term (hunger or thirst... getting sick... a broken heart), others long-term (mental illness, being handicapped). And there is also the kind of suffering that comes from being human, such as aging.
Your mind is either at peace or it is not-- many of our actions stem from this. If you are content with your life, the happiness you experience radiates in your interactions with other people. But if you are suffering, if you feel that something in your life is amiss, your interactions with others are more likely to be negative. How can you be kind to the world, if you feel it's unkind towards you? You don't make war if you are at peace; you don't resort to self-destruction.
Another thing that's worth taking into account is someone's background or upbringing, because your childhood shapes how you later interact with the world. Experiencing love early in life helps us return it to others later on.
So when I question what motivates someone's actions or thoughts, I ask, "Is he/she content with life?" And I relate my experience in trying to understand other people. We are all not so different, after all.
Lessons & tips: Try to understand yourself first; understanding others will follow.
Resources: experience, reflection, self-awareness
