We see immortality as something worth having because it means we could not worry about what we eat or what we do to our bodies or how much we smoke or drink. You live forever, but your relatives and friends do not. They die off like normal humans, leaving you completely alone and without any ties and leaving you feeling like you betrayed nature and your family line. Mortals are always concerned about going out and doing things and collecting experiences before they grow old and they can't move anymore. If you're immortal, there is no reason to do that.
Immortals have no time of reference, really.
Everything falls into the present. You aren't one segment of the timeline. You are the timeline. In short, your existence loses all meaning. Nothing means anything, if everything is the same.