Immortality - or even just an extremely long ‘normal’ lifespan - might sound amazing on paper, but it’s not always all it’s cracked up to be: Sure, getting to experience anything the world has to offer without fear of death might be fun at first, but there will come a time where you’ll have tried it all; every activity done, every place visited, every skill learned, every type of person met.
Eventually, at some point, you just won’t be able to get excited for or get any fulfillment from anything anymore - eventually, it’ll wear anybody down: You'll be a walking husk numbed out of all feeling, wishing for that ‘spark’ of life that made everything so vibrant to come back to you.
This fixes that issue; with this perk, you’ll never lose that ‘spark’ again, allowing you to experience whatever the world has to offer as much as you want without ever growing dissatisfied or listless, each time feeling even as fresh and new as the first if you wish. This does not affect your personality, either, only making sure that you'll never be susceptible to ennui and listlessness unless you wish for it.