I'd say Immortality/super regeneration. Sure, your body and organism never deteriorates, and instantly regenerates, but while you live on, you watch your loved ones grow old and die. Plus, if you already lived for a couple of hundred years on earth, you witnessed enough wars, epidemies and natural disasters to give you a monstrous PTSD, with which you need to live forever. Just look at our boy Logan here. Is he immortal? Practically, yes. Is he happy about it? Absolutely not.
Do you like your peanut butter smooth, or crunchy?