A Loss of Income
When two people get married at a young age, they may spend decades planning their retirement. It does happen at times that one person will pass away soon after they have retired. Their spouse may be able to continue collecting part of the pension the deceased spouse spent many years earning, but it could result in a loss of income if they eventually choose to remarry. This is often a stipulation in pension funds, and it has long been a reason for mature couples to opt for living together without the benefit of marriage.
It might seem to be a cold decision to live in this way when two mature people are in love, but it should be noted that many spouses are willing to go to great lengths to help a partner throughout the years of their marriage. Giving up time and putting in effort at home so a spouse could earn that pension means it was part of their retirement plans. Giving it up simply because a spouse is gone is about giving up that investment in a lifetime of sharing.