It seems to me that most of us become more pessimistic as we grow older, and I presume this would be the case if we could live forever. Probably, living for ever would make people blasé and bored - after all how many new experiences can there possibly be after the age of 100 and something?
I would not like to live in a world where there were no, or only very few, children and young enthusiastic people, which is what might well happen if we could live for ever.
I frankly don't believe it is possible to extend the cellular life of the human body indefinitely.
The over-population that many of you have already mentioned makes me afraid that if people could live forever, some kind of culling in the worst and most scary science fiction senarios would be instituted to get rid of the surplus.
Not a happy thought, is it?