Wolves, swans and French angelfish are among a litany of species which mate forever. If a partner dies, the survivor literally becomes a lost soul and gives up all interest in continuing alone.
Humans, too, often embrace the mindset of there being only one true love allotted to each of us per lifetime. In Eve's Diary by Mark Twain, an inconsolable Adam weeps at her grave with the words, "Wheresoever she was, there was Eden." Even if Eve had been the first one to face the future newly single, there'd have been no Central Casting to which she might easily apply for a replacement.
In a perfect world, we would blissfully exit our shared existence hand-in-hand. In an imperfect one such as that in which I suddenly found myself after 25 years of marriage, we can either fold to despair like a cheap suit or stride boldly into grand adventures and do our dearly departed lovers proud.
My vote is for striding boldly. Who's with me?