There's actually a wonderful insight into human evolutionary psychology here. We've evolved to discount the future. We are finite beings and all of our eggs are in one basket. For a large part of our biological history, performing beneficial tasks such as eating or mating was best performed in the present, because you might not be around next month to do them. There might be a famine, or you might get injured, thus causing your prospective mate to choose a healthier candidate. Courses of action that result in benefits now (or the prevention of an immediate loss) are much more compelling than courses of action that have a benefit in the distant future.

There's a great website about procrastination, http://www.procrastinus.com/, that uses this understanding of future discounting to help one get things done. The idea is that a long project should be broken up into smaller tasks so that, even though meeting the smaller goals results in less absolute reward, they are more compelling because the completion horizon is so close.

Basically, humans are wired so that a jelly donut (or a few hundred bucks) now is perceived as more beneficial than 5 to 10 years of existence tacked on at the end of life. For a very long time, that has been a very sensible proposition.