The Second law of thermodynamics states that the overall entropy of the system increases over time. Entropy is, more or less, energy dispersal. It is the reason a glass of ice water will eventually reach the same temperature of the room around it.
In his book Enlightment Now, Steven Pinker uses this law to claim that even if scientists are able to prevent cell death and therefore increase the average lifespan of humans entropy will be there to affect whatever prevention exists. Coupled with the fact that random, unforeseen events could cause death and it becomes clear that perpetual immortality is not possible.
