I have found that it is dangerous to assume that others are essentially like oneself.
Psychologists speak of projection. As I understand it, it involves projecting into others one's own attitudes, beliefs, motivations, fears, emotions, desires, values, and the like. It is classified as a defense mechanism. Suppose one is stingy, considers stinginess an undesirable trait, but doesn't want to own up to one's stinginess. As a defense against the admission of one's own stinginess, one projects it into others.
I once had a superficial colleague who published a lot. He was motivated more by a neurotic need to advance himself socially and economically, a need based in low self-esteem, rather than by a drive to get at the truth or make a contribution to his subject. He was at some level aware that his motives were less than noble. Once, when he found out that I had published an article, he told me that my motive was to see my name in print. It was a classic case of projection: he could not understand me except as being driven by the same paltry motives that drove him. By projecting his motives into me, he warded off the awareness of their presence in him, or else excused their presence in him on the spurious ground that everyone has the same paltry motivations.
Most of the definitions of projection I have read imply that it is only undesirable attitudes, beliefs and the like that are the contents of acts of projection. But it seems to me that the notion of projection should be widened to include desirable ones as well. The desire for peace and social harmony, for example, is obviously good. But it too can be the content of an act of psychological projection. A pacifist, for example, may assume that others deep down are really like he is: peace-loving to such an extent as to avoid war at all costs. A pacifist might reason as follows: since everyone deep down wants peace, and abhors war, if I throw down my weapon, my adversary will do likewise. By unilaterally disarming, I show my good will, and he will reciprocate. But if you throw down your weapon before Hitler, he will take that precisely as justification for killing you: since might makes right on his neo-Thrasymachian scheme, you have shown by your pacific deed that you are unfit for the struggle for existence and therefore deserve to die, and indeed must die to keep from polluting the gene pool.
Projection in cases like these can be dangerous. One oftens hears the sentiment expressed that we human beings are at bottom all the same and all want the same things. Not so! You and I may want "harmony and understanding, sympathy and trust abounding" but others have belligerence and bellicosity hard-wired into them. They like fighting and dominating and they only come alive when they are bashing your skull in either literally or figuratively. People are not the same and it is a big mistake to think otherwise and project your decency into them.
I said that the psychologists classify projection as a defense mechanism. But how could the projection of good traits count as a defense mechanism? Well, I suppose that by engaging in such projections one defends oneself against the painful realization that the people in the world are much worse than one would have liked to believe. Many of us have a strong psychological need to see good in other people, and that can give rise to illusions. There is good and evil in each person, and one must train oneself to accurately discern how much of each is present in each person one encounters.
One mistake I have made, more than once, is to assume that since I value truth above many other things, others do as well. But there are plenty of people who do not value truth at all, or else assign it a rather low priority. There are many, for example, who value human feelings over truth. ytruth s nothing to them; feelings everything. That makes no sense to me; to me it is self-evident that, although both are values (to be precise: things that ought to be valued), truth is a higher value, if not the highest value. But reality forces me to accept that others hold to the opposite value-prioritization. It is folly to project one's own values into others.
There are other people for whom truth counts for nothing, but power for everything. They interpret every type of interpersonal transaction as a power struggle. Thus if you calmly try to persuade such a person of the truth of some proposition by appealing to facts and reasoning correctly from them, he will interpret that as nothing but an attempt to dominate him psychologically. Such people are utterly blind to the value of truth and to the fact that truth can sometimes be attained by dialectical means. They project their own lust for power into everyone else interpreting everything that is manifestly not a power-move as latently a power-move.
There are plenty of leftists like this. Taking their cue from Nietzsche, they assume that everything is power at bottom. Die Welt ist der Wille zur Macht und nichts anders! "The world is the will to power and nothing besides!" Supported by this assumption, they set out to unmask (deconstruct) phenomena that manifestly are not power-driven, for example, attempts to state what is the case. Power-mad themselves, these leftists project lust for power into everyone and everything. It is a curious pars pro toto fallacy: one takes a phenomenon one finds in oneself, lust for power, and then interprets everything else in terms of it. The idea might be worth exploring that Nietzsche's doctrine of the Will to Power arose by projection. He saw the lust for power within himself and excused its presence there by projecting it outward thus transforming a psychological peculiarity into a fundamental trait of beings qua beings.
You say I'm psychologizing. True enough. But false views are legitimately psychologized. It would be the genetic fallacy to dismiss as false a proposition because it arose from a need or serves a need or results from projection. But once a proposition has been shown to be false, it is legitimate to inquire into the genesis of the belief that the proposition is true.