Humans are a social animal, so the aspect of being alone or the factor of loneliness is counter to how humans are naturally wired.
We learned to hunt and watch the offspring as a group in the caves and fast forward and this is us, we work together and want to be together.
Loneliness being addictive? That depends on one's definition of addictive. For me it comes down to habit and if one gets in the habit of being alone too much it becomes natural to that person.
Good for a person to be by themselves? NO WAY and medical has time and time again said this.
All through the decades that I have been on this planet, I have worked and dealt with folks that were single and after a little while, I could pick out in a few seconds of meeting someone if they were single and especially had no social support network. Thier mannerisms, their body language, everything, mainly the way they carried themselves with others. Same with me, I can tell when I get in a crowd that I feel less at ease, and I sometimes do not know how to carry myself in a group.
Walter