Freud writes in Civilization and its Discontents that when we're first born, we recognize little separation between our egos and our surroundings. However, we soon learn -- unless we have certain psychological pathologies -- that we're distinct entities. We notice that we desire things that aren't contained within our bodies, so we adopt the reality principle and sever our egos from our environment.
"Maturity" isolates and shrinks our egos, but not irrevocably. The act of sharing re-awakens our more interconnected selves. It blurs the boundaries between what is mine and thine, as philosopher Win-chiat Lee so eloquently explains; and it brings us together in the process.