One of the greatest psychological follies of man is that we assume everyone else is like us. Not consciously, but subconsciously, we incorrectly project our desires and way of thinking on everyone from family to strangers.
This leads to the almighty miscommunication and also explains how people are taken advantage of and how people can take advantage of others without regret.
If you are generally a selfish person who is cut throat and "does what has to be done" and doesn't mind making a few enemies and burning a few bridges to get what you need - you will tend to assume that others are that way too. It becomes easy to justify your way of acting and thinking with phrases like "they'd do the same to me if the situation was reversed".
If you are generally a kind person who gives others the benefit of the doubt and tries to accomodate others - you will tend to assume that others are that way too. You will go out of your way to help and lend a hand because you assume that others would do the same for you. Then, when they don't. Or, when they act ungrateful for your sacrifices, you will be genuinely hurt or confused.
Try to wrap your mind around the fact that everyone is unique. And, even on your best and most intuitive day, you will not be able to guess what they are thinking - how they are motivated - or what makes them tick. This is when the phrase "This, above all, to thine own self be true" becomes a mantra that will serve you well.
Because, you - whoever you are - is the only person you will ever even hope to come to understand. And even that isn't certain.
Attempts at understanding others are often futile and often lead to great disillusionment. Trying to understand others and base your behavior on your predictions of their behavior is a recipe for your own pain because it will often require that you go against who you are at your core.
On the other hand, if you live your life authentically - there are some things I can guarentee...
1. You will hurt others accidentally
2. You will please other authentically
3. You will be able to sleep better at night knowing you were sincere in both actions