An innate part of humanity is our capacity to change: over time as individuals, from generation to generation, following major historical/technological/scientific upheavals, etc. Part of these transitions involves the death of generational viewpoints, specifically through the death of individuals.

If I had the choice to live forever, I assume other people (or everyone) would be given this option as well. Us immortals would just get in the way of the future, probably develop some elitist statuses and might become cripplingly permanent connections to obsolete traditions and views. Think about it -- Thomas Jefferson was amazingly ahead of his time, but he still owned slaves.

An inherent part of life is it's opposite: death. Without a thought for the unknown oblivion beyond some decades of existence on earth, what would our lives be worth? With an eternity of possibilities extending in front of us, wouldn't everything be reduced to triviality? If given the option to live forever, I'd decline, opting for some definable number of years filled with certain excitement and meaning, knowing they could end at any minute.

It's really cliche, but I also believe in paving the way for future generations and their ideas, allowing for the evolution of society rather than the stagnation that would most likely come from widespread immortality.