at different times we crave different things, young adults may have a lot of social pressure to be surrounded and liked (maybe a pre-sexual builtin to ensure being part of a pack) that can cause sensation of loneliness through a feeling of not being normal.

I, and a few others, felt that urge to fit in, until a point where your brain just matures and says 'fuck it, I know what I need, and "others" may not be that, or rarely so' which makes being alone a lot less of an issue.