Imagine it differs per person, as with pretty much anything about us, a spectrum.
Would say it is mostly about having a real human connection filled with love and the good feels. Instinctual, comparable to how many women feel about ‘needing’ to have children. Consider it a base requirement for a fulfilling life, like shelter, food, water, air.
Some people appear to have enough loving only themselves in that way (or not at all); perhaps they just have not yet found the right person. Of the many ways to express love, this is one of the most basic.
The best of us in the hands of the worst of us.