Leviathans are possessive beings. If there's something they want, they will have it no matter what the cost. There was little that defied the Leviathans anyway. But there was one thing, that even the leviathans, with all their might, could not prevent.
Death took what was his, paid no heed.
And the leviathans were painfully aware of how short the lifespan of Mers was. Blink once and all their treasures would have turned to seafoam as they had joined the ancient Titan of the Seas. And like it was said, the leviathans were possessive. For while they knew it was a great honor for any mer to join their ancestors in the seafoam, none of the leviathans wanted to hear their beloved's roar alongside that of the megalodon in the seafoam.
So the leviathans did something they had only heard, that it was possible - they bondet their souls with the ones their mates, giving them the same lifespan as the leviathans had. It wouldn't be stopping death forever from taking them, but at least the leviathans could keep them by their side longer.
Once part of a leviathan's treasure, always part of a leviathan's treasure.