First, what you feel, other people would feel; what you fear, desire, despise or admire, other people do. If you do not believe me read the Essays of Montaigne  and you will see yourself in a mirror.
With this recipe of knowing people you keep things simple.
Second, the differences - and there certainly are differences - come from personal histories, circumstances, roles, identities, interests and points of view, which you can also find out or recognise if you care to inquire and observe.
Start with asking yourself what you would do if you were them, in their situation, then look, listen and judge with common sense, as things keep changing. You will become a fine psychologist. Forget the complicated theories.
Basic things push people, or pull them to do what they do. First things act first: fear of death, urge to be safe of pain, fear and loss, need to own things, certainties and territory, a place among the other to belong, to communicate, sex, pride, desire for power, hope, being who you truly are, ever better  and later if not last, being civilized and good, spirit divine - other than beasts. Very rare are the perverts with an urge of death and a love for evil for evil's sake.
 Essays of Michel de Montaigne, Tr. Charles Cotton, 1877
 Maslow Abraham, Motivation and Personality, Harper & Row, 1954
 Maslow Abraham, A Theory of Human Motivation, Psychological Review, 1943, Vol. 50 #4, pp. 370–396.