A personality, or psychic system, shapes itself through reactions to all sorts of external stimuli, including the presence of other people. There are three possible reactions to things in the world: indifference, sympathy, and antipathy. According to Simmel, the neutral reaction of indifference is rare. Our feelings of sympathy or antipathy can be so subtle that we might consider them neutral, but if we were drawn into closer proximity to the thing we would likely feel more overt sympathy or antipathy.
If a psychic system responded the same way to everything, which would mean feeling indifferent to everything, repelled by everything, or attracted to everything–the personality would remain the same; it would never change or evolve (implying absence of autopoiesis). But because I am repelled by or attracted to a particular person, architectural design, book, piece of music, etc., I develop a unique personality. I become who I am by loving some people, admiring some works of art, detesting some people, being disgusted by some foods, etc.
This is all about affect–my conscious, semiconscious, or unconscious feelings about whatever I encounter.
We might extend this principle to all sorts of systems. A married or long-term couple tends to share a collection of likes and dislikes. The same goes for organizations. Organizations are often built around common likes or dislikes–or passions.
Simmel also says that conflict–rather than causing tension–resolves tensions. Thus, conflict is necessary for the continuation of social systems. A marriage or other important relationship, in this view, should not be completely peaceful. Expressing anger can also allow a person to avoid feeling helpless or dominated by some powerful entity or force.
This all goes back to Luhmann’s idea about a society not being held together by love, harmonious cooperation, or consensus. All that’s actually needed is communication. Two people who continue to communicate despite their disagreements or arguments will continue to exist as a social system. Argument counts as communication.