Humans have a really hard time imagining ourselves in the future and altering our behaviors and expectations accordingly. Most of us would rather experience pleasure in the current moment, while leaving the pain for later.
This is a bias that is of particular concern to economists (i.e. our unwillingness to not overspend and save money) and health practitioners. Indeed, a 1998 study showed that, when making food choices for the coming week, 74% of participants chose fruit. But when the food choice was for the current day, 70% chose chocolate.