TLDR:

When school, work, gaming, art, reading, and basically the rest of your life is done on a computer at a desk then you start to get tired of sitting after a while. I built a program to remind me to stand up every 45 minutes, so I don't go entire days without standing.

Simple Health Science:

Caveman Science: Humans not made for sitting. Sitting bad. Standing good.

Realer Science: According to the University of Waterloo, "Sitting behind your desk all day is bad for your health and experts have long been advising people to stand at their workstations for about 15 minutes an hour. But a University of Waterloo professor says his research shows that people should be standing for at least 30 minutes per hour to get health benefits."