To a first approximation, the left is generally optimistic about how nicely people would behave if only they were given the chance, and how much scope we have to decide how our societies should be so ordered as to give people as much of a chance as possible to be nice. The right is pessimistic about both of these things: people have as much of a predisposition to be nasty as to be nice, and societies develop organically along fairly constrained paths in which such dispositions, along with external resource constraints, are strongly limiting factors. The right tends to overrate how much you can put right through the imposition of top-down authority; the left tends to overrate how much will go right by itself if people are freed from artificial limitations.
There is a kind of diagonal hack through this standoff, the "true knowledge" approach, which is maximally pessimistic about human nature, but reasons from there to the necessity of something like socialism as the only system which adequately disarms and tempers the human propensity to accumulate power, lord it over other people and get endless kicks out of gratuitous cruelty and humiliation. Rational self-interest, pursued with maximum tenacity and foresight (rather than just trying to score easy wins in an endless war of all against all) leads to communism because everything else ends in war and mutual destruction...