Our bodies don't make the decision to weaken and die, they do everything they can do to maintain, until they can't anymore. It's the outside environment combined with the physical matter our bodies are made up of that causes it to eventually die.
All of the things that we use to maintain can be regenerated in the same way they were generated in the beginning, so obviously they can be recreated to replace the expired ones.
Think of it like a second puberty (talking about eating habits). When you run low on supplies, you grow a new psychological compulsion to gather the necessary materials to fix yourself up.
We don't have that phase in our lives so there must be a reason. It's possible that rearing children dooms us: Self neglect in the face of caring for the vulnerable young pushes us past the point of no return.
That is to say, the degeneration that inevitably happens during child-rearing and the acceptance of it (which might even have been ingrained parallel to young-nurturing drives as we evolved) probably do enough damage that regeneration of maintenance systems becomes too difficult -- a maintenance growth spurt is just too exhausting to acquire the resources for after all the compromised systems.
After all, the physical faculties are quicker to go than any internal systems, thereby making an overhaul more expensive than profitable in terms of energy input and output.
Imagine, by analogy, a dying company who's run themselves out of the means to purchase new equipment which happens be the only potential salvation of the company.
They can't save themselves.
The only other explanation I could come up with for why we die hasn't happened yet, so it doesn't make sense but it will happen and it makes me laugh.
Overpopulation is a good evolutionary reason to avoid living forever.
Huh... when I started writing I thought I agreed with Jock, but I don't.
'Course I wasn't really... there... when we evolved so there's undoubtably something in the chronology that I missed, but I imagine the story went something like this.
The thing is though, my impression is that it only originally began this way, and as we evolved, it became more of a psychological acceptance of death, which blocked reason to ever develop a drive to survive to an overhaul.
So I guess I kind of do agree with him.