I don't believe in some kind of predestination either, I have found that for me, friends come and go. Some you have known even for years sometimes move on, and sometimes that also leads to you not having any if it just so happens that everyone else got married, kids etc and you are still at the same stage. If you suffer from anxiety then that can also mean making friends can be very hard, causing your friendship group to slowly decline until you are alone. That's kind of what happened to me. I always knew that I was kind of different, I was liked by lots and lots of people... but never loved. Always the guy people were happy to have along, but not the one who is ever actually wanted.
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I agree with this. When you reflect on it the very fact that you are more alone than others has its enviable points too. You have more time to relax. Be less pressured to be up til 2am, or imbibe in vices like drinking or smoking because you are not as 'popular' and more 'rejected'. Some of these so-called joys of life may pass you up and you may actually be better off for that. More time for healing or sensing a need to heal, on both a physical and spiritual level.
I evaluated the lives of more extroverted (not necessarily happy) people and these are the conclusions I had found. Not saying this is going to be everybody's finding but that was my take.