Leftist psychology is simple and extremely effective: it has to be simple, because it is almost universal; it is almost universal because it is so effective.
(In The West, almost all supposedly Right wing people, and all in the mainstream, are merely variants of Leftist - just a small minority of religious people are Not Leftist.)
There are two aspects to Leftist motivation - one is self-interested, and the other is what provides a veneer of morality. Neither would work well alone.
If Leftism was wholly self-interested, it would not be a success - the key to Leftism is that it enables people to be self-interested while also feeling good about it - feeling, indeed, altruistically superior about it. But if Leftism was genuinely altruistic, self-sacrificing at an immediate and personal level, it would not be popular.
In the first place, Leftism creates, mobilises, amplifies resentment - class war, war of the sexes, race hatred, the resentments of the sexually non-normal. Since resentment is the commonest, and most insatiable, human sin - this formalisation is very powerful. Few are immune to the blandishments of victimising one's enemy.
Resentment is the self-interest that defines the acceptable enemy, the scapegoat - he or that who is (as an individual or group) to be stripped of status, power, resources. Leftism is based on identifying such victims, asset-stripping them, and using the assets to buy and pay supporters (keeping aside a share).
Leftism makes this act of coercive violence into an act of altruism, by its various (often elaborate) theories - at bottom, these all amount to the idea that it is always good to take from the evil, and give (some of) what is extracted to the virtuous.
Over time, Leftism has varied, sometimes reversed, the definitions of evil and virtue - but those who are, by one definition or another, virtuous; can expect rewards from this extraction, and have a direct self interest in one or another version of Leftism.
Furthermore, Leftism has developed a powerful way of justifying, morally - and in daily practice - being short termist and selfish; while justifying this in terms of displaced, remote, theoretical long-term altruism. Again, this is the doing of theory, of abstraction - which is the job of Leftist intellectuals.
Leftism is about self-hatred - and this is very important; indeed vital to its long-term success. Self-hatred is the deep morality of Leftism.
This means that the Leftist leadership can convince themselves of their own altruism by the fact that they will often argue against their own long-term, group interests.
Upper class Leftists will designate their own class as evil (almost all Leftist leaders, past and present, come from from the upper classes). Men will be feminists. White Native-born people will denigrate the white native population. Married people with families will advocate sexual revolution... and so on.
It is this self-attack, this suicidal self-hatred, which acts to reassure the Leftist of his own high moral values - and proves (to himself, at least) that he is not merely self-interested.
Yet - and here is another vital aspect - the suicidal self-hatred is displaced. The Leftist attacks his own class, sex, race - and designates the group as an evil exploiter - but not him-self. He asks his class/ sex/ race to give-up their status, power and wealth - but not himself... at least not-here and not-now.
The Leftist leader displaces the sacrifice away from himself and into the future, and often other places. So the Leftist leader's mantra is Not me, Not here, Not now... Arguing that "People like me should give up their jobs to minorities - but not me". Or, the leader argues that a working-class person, woman, an immigrant should have jobs in preference - but this does not apply to my job, here and now.
Always the sacrifice is distal while benefits are proximate - Not me! Not yet!
(The usual excuse is that "I be allowed to keep my status, power, wealth for a bit longer - until I can finish this important work of Leftist redistribution". For a Leftist leader; when-utopia-arrives, when The System is just, is the best time to yield privilege; before then it would be futile or premature... Thus all Leftist leaders are hypocrites, all are Quislings, all are traitors and sell-outs - it goes with the job description.)
So this is the psychology that has led to Leftism taking-over the world. It is selfish - which is why it is so very popular; and it is moral - which makes it even more popular.
But, crucially to Leftism's success, the selfishness is proximate, direct, immediate; while the altruistic, sacrificial morality is abstract, impersonal and remote.
Note: All the above is necessarily and intrinsically a feature of Leftism, because it is this-worldly, and denying of God and creation. If you think about it; in a world without meaning, when death is extinction, morality can only be some variant of the above.