Been married three times, still with wife #3
Wife #1 was killed in a car wreck, so I remarried quickly- (on the rebound) because I had a one year old son and wanted to put my life back together again. Made the mistake of marrying the wrong person, it lasted 8 years till she ran off with her old boyfriend. But I got a great daughter plus two stepdaughters out of the deal and I really love them all.
Got some good friends who prefer to be single, they have more time to do thier own thing, not have to take into account another person's preferences or feelings, come and go as they please, etc. As we age, we get more set in our ways and you know it's hard to teach an old dog new tricks
Some folks like company and a companion, other folks seem content to be alone. My favorite aunt once said "it is better to be lonely and alone then lonely and with someone".
If you have kids/grandkids, and/or lots of relatives and friends then old age is not so bad alone. But if you don't have a good social support network then perhaps it leads some to seek a more permanent companion so as to fight off the loneliness. But some folks just seem happier alone then with someone.
Didn't Three Dog Night sing about 'one is the loneliest number' . . . . .