2) It would make relationships with other people extraordinarily difficult.
Affection is responsible for nine-tenths of whatever solid and durable happiness there is in our lives.
If that’s true, how happy would an immortal be? As you get older, some of the deepest hurts are the losses, the people you cared about deeply who are no longer in your life. Imagine the impact of those losses stacking on top of each other, one after the other, decade after decade, century after century, millennia after millennia. Could you go anywhere, do anything, without it reminding you of someone you cared about whom you outlived? Imagine being immortal and finding your soulmate. Then, year after year, she’d grow older as you remained the same age. Eventually, you’d be a 25-year-old man in a relationship with an old crone. Even if you were okay with it, can you imagine how weird and awkward it would make her feel? What a depressing existence that would turn out to be.