In some British (if I'm not mistaken) TV show there was a dialogue on this topic. Very beautiful. The girl was asked who she considers herself: introvert or extrovert. She replied that there was a little bit of everything in her. She is an extrovert because she loves big companies and communication in them, but an introvert because it’s not true.
But in fact, the question has long outlived itself: the fact is that such people may not think about anything at all, they may think about some nonsense or about what to think about in a decent society shouldn't.
In general, the question was initially posed incorrectly, it should sound like this: why people who are not rejected by society are more comfortable alone. If the question were posed like this, the answer would be from the opposite: why, in fact, are all people not comfortable in such conditions? If you delve into this topic, then you can come to the most amazing conclusions, but, in my opinion, this is superfluous. The best thing would be if the person who is interested in this asks himself these questions, answers them as honestly as possible and draws his own conclusions. Usually they amaze with their surprise. To the person who answered these questions, the understanding of what they are thinking will come independently.
What are they thinking? I will not say that I am one hundred percent who you are asking about, but I belong to that number, more than half, and based on my father's opinion, this is practically everything, without exaggeration. But it all depends on the hobbies of the person. So I love economics and psychology, there can be a corporate and brawl around me, I can sit and watch everyone calmly, just because I am interested in psychology, I can think about paradoxes, solve some riddles, about economics in the end ... There are also generally funny thoughts, such as an ideal dialogue in any situation, or an analysis of a past situation, the realization that if it were not for emotions, how everything could have been done better. Sometimes it looks like you are sitting in the middle of the room and millions of fish are swimming around you, and they are all your thoughts, you think about many things at once and then someone or something enters the room and all these fish disappear in a moment, you try grab at least one by the tail, but even. I often think about the future or the past, in general, you can write until morning. One thing I can say for sure, if such a person does not get bored, then he thinks constantly and his thoughts make sense. For those who are bored start looking for a company.