There are many reasons why a person ends up being alone. Some of them are negative, and some are positive, and some are accidental, and some of these choices happened "accidentally on purpose."
Sometimes you make choices to do what you think you want to do, and only later find out that you've inadvertently chosen to do it all by yourself after you're entirely committed. If you think about how long it takes to get exactly what you want, how long people work to set themselves up for retirement - it's a long haul. Most people don't know what they're going to do with themselves when they retire because they never had a chance to do what they loved to do - or they're not able to reinvent themselves.
Sometimes it is just that we cannot "sell" to others our desire to have things the way we want. There has to exist someone else who wants share that situation with us, and that is sometimes a tall order if our dreams are unique. Or sometimes, we end up alone because there is something we do that is not socially acceptable to others who would usually want to hang out with us - too much "baggage."
I am overwhelmed with how many of my peers have become alcoholic as they age - probably somewhat because "it doesn't matter now." Some people go through a process of "giving up" (sometimes necessary) without ever coming out on the other side of it to understand what they positively do want. Some just decide not to bother learning anything new and get along with what they have. For them, it's just too much trouble to feel like you don't know what you're doing. This is one example of circumstantial isolation. If you don't change with the times, you get left behind. And possibly disillusioned about what you believe is not possible now.
Relationships teach you about yourself. (At least, that is how they have always worked for me.) In a way, it's expending less effort to be alone. All those character defects that are irritating to someone else don't matter when you're alone. Nobody else is there to accept the consequence except you. If you're OK with what you do, you can have things be any way you want. Of course, some older folks get very "set in their ways." But think of the advantages, you don't have to explain or apologize about the way you are, how you work, why you do what do. There is nobody to answer to. Being alone is both an advantage, and a disadvantage.
Someone asked an old woman past 100 what is the best part of being old - and she said, "No peer pressure."
Lightly skipped over by most is underestimating the power of a significant booby prize that is the strangest feature of being lucky enough to get old. Watching those twenty, thirty, forty year friendships dissolve or suddenly disappear is not for sissies.
I don't know what your experience with death and the process of grieving is, but in my peer group - people do not want to hang around someone who is grieving. Oh, after someone dies, people will gather to console one another and commemorate, but then, they go away. They don't call, they don't make contact again for another...three months or so. Then it's only a light check-in.
Somehow the person who is grieving is supposed to "get over it" or "deal with it." Most of your friends will get tired of hearing about how you feel when you miss someone who has died. This distaste perhaps reminds them of how they have trouble thinking about their own death. Maybe it's just too close to how unpleasant it would be to be the one who is left after best friends and lovers are gone - as you are now. It's as if grief is a state of social quarantine.
There isn't much social support either for accepting each person's unique process of going through their own grief. Instead of recognizing that grief is a special time of self-knowledge, if the process of grieving goes on "too long," it is labeled "depression."
It's not depression, it's grieving - but this is the cloak that society wraps it in. This mistaken label for grieving as "depression," is yet another very good reason for circumstantial isolation.