I never quite put it together until recently, but relationships of all kinds can be really exhausting. Naturally I tie this to autism to some degree since my diagnosis, but it was always there in front of me when I would talk to various therapists. The pattern was pretty clear, and it didn't matter if it was romantic, friendship, professional, family, work... there were always times when I needed to "recharge." That term gets used a lot by therapists, and it's hard to embrace it as something other than a character flaw. But it makes total sense.
Adding the ASD dimension to it, I think there's a lot of nuance to pile on there. Relationships of all kinds come with about million different social contracts, and if there's one commonality that's fairly consistent about the autistic, it's varying degrees of non-interest in social contracts. These are often labeled as social impairments, but are they really? Let me give a silly example. My entire life, I've found formal clothes, suits, ties, to not serve any legitimate purpose. Sure, people will talk about it in terms of status, respect and the like, but these are all arbitrary customs that have festered into "normal" without anyone really questioning them. This one is reasonably innocuous, but other arbitrary customs that have been normalized range from the severe like slavery to the truly illogical like the taboo of seeing a woman's milk dispensing anatomy. For the record, I find all of the above pointless (and outright immoral for slavery).
Now think about all of the social contracts that come with any of those relationship types I categorized. The romantic contracts aren't even the hardest ones, and if you land with someone who really gets you, they might even be the most effortless. Friendships can be pretty exhausting, which may explain why so many of the people I value the most as friends are people I generally see once or twice a year, at best. Family, man, I don't even know where to start with those, as those seem to be the root of most needs to see a therapist. I had a partner whose family love was completely predicated on the scope of the gifts you gave at Christmas and birthdays. You can imagine how well I fit in there when I'm not that interested in either of those, let alone any kind of score keeping. As an individual, I see professional relationships as straight-forward, I perform a service and you compensate me for it, but as a manager I have deep understanding of the complexities of intrinsic motivators and interactions between people to get to certain outcomes. Oddly enough, most of that comes naturally to me, but it's definitely exhausting at times. Managing up is even harder, because I've never trusted any boss, ever. I don't know if that's true for other, non-autistic managers, but it's something I should explore. But even then, I've achieved great outcomes and been dismissed because I didn't fit the social contract mold, for reasons I may never understand beyond a non-interest in hanging out socially.
Similarly, we often want to group people into neat little piles of introverts and extraverts, dismissing the possibility that ambiverts are a thing. I can be quite extraverted when the situation requires it of me, but I'm not even kidding when I say that I need a day to recover from it. Again, I can excel in these situations, but I'm quite content also to just be by myself, or with my mate, and leave it at that. I love having people over, making food and drink for them, but I need breaks. When my in-laws or extended family visit, it's great to see them, but I definitely disappear for periods of time to have a quiet moment to myself.
There's an interesting dynamic between the need for human contact and the cost of it. I don't know that this is true for everyone, but it certainly is for me. I spent my teenage and college years desperately wanting to belong and be loved. I never did quite belong, and I made peace with that, but the love did come. Those relationships have been intense, and I'm grateful for them. I don't think I could sustain a high level of engagement with even a half-dozen people though. It would be too much. I am grateful to have a partner that accepts who I am, and probably puts up with more shit than she deserves because I don't recognize the shit.
Keep this all in mind the next time you encounter someone who seems antisocial, whether it be someone you meet, work with, or even a family member. What you perceive as aloof or selfish may be completely wrong.