I think that people behave differently in defferent situations sometimes because they don't want to hurt people around them and sometimes they just don't care.
Sometimes people are just afraid of the reactions of others.
Not everybody has the same reaction to the same situation, we don't all think alike, so it can be tricky.
Don't expect something if you, yourself is not ready to give, and don't do to others what you don't want people to do to you.
I once read a really interesting comment about this. Sadly can't remember the author, but she said "we judge ourselves by our intentions, and judge others by their actions." Her example was punctuality; if we are late ourselves, we feel we have a good reason for it, and expect others to be understanding. However if another person is late and keeps US waiting, all we think is that they can't be bothered with us, or something else is more important to them than we are.
This cleared up a lot of things for me - we know our own motives and mostly we don't mean any harm, so I suppose that's why we always expected to be judged as well-meaning, harmless poeple - whereas if it's someone else, if they do any harm we tend to assume they did it on purpose.
Anyway, that's the best explanation I've found.
Personal perception and evaluation in fact vary greatly between individuals. The repeating of actions previously successfully employed by a person, become a habit. If each case is not prioritized the exact same manner and adjustment latitudes have not been standardized. Each instance has the possibility of being down differently each time!