We backslide because it's hard to fight habits. 

On homeostasis:

As George Leonard discusses in his classic book Mastery, based on his experiences in the patient lifelong practice of Aikido, it’s not necessary to beat ourselves up or derive a complicated psychological explanation.

The problem is due to a very simple mental model that explains how systems are regulated through feedback loops: Homeostasis.

Backsliding is a universal experience. Every one of us resists significant change, no matter whether it’s for the worse or for the better. Our body, brain, and behavior have a built-in tendency to stay the same within rather narrow limits, and to snap back when changed—and it’s a very good thing they do. Just think about it: if your body temperature moved up or down by 10 percent, you’d be in big trouble. The same thing applies to your blood-sugar level and to any number of other functions of your body.

This condition of equilibrium, this resistance to change, is called homeostasis. It characterizes all self-regulating systems, from a bacterium to a frog to a human individual to a family to an organization to an entire culture—and it applies to psychological states and behavior as well as to physical functioning.

The simplest example of homeostasis can be found in your home heating system. The thermostat on the wall senses the room temperature; when the temperature on a winter’s day drops below the level you’ve set, the thermostat sends an electrical signal that turns the heater on. The heater completes the loop by sending heat to the room in which the thermostat is located. When the room temperature reaches the level you’ve set, the thermostat sends an electrical signal back to the heater, turning it off, thus maintaining homeostasis. Keeping a room at the right temperature takes only one feedback loop. Keeping even the simplest single-celled organism alive and well takes thousands. And maintaining a human being in a state of homeostasis takes billions of interweaving electrochemical signals pulsing in the brain, rushing along nerve fibers, coursing through the bloodstream. One example: each of us has about 150,000 tiny thermostats in the form of nerve endings close to the surface of the skin that are sensitive to the loss of heat from our bodies, and another sixteen thousand or so a little deeper in the skin that alert us to the entry of heat from without.

An even more sensitive thermostat resides in the hypothalamus at the base of the brain, close to branches of the main artery that brings blood from the heart to the head. This thermostat can pick up even the tiniest change of temperature in the blood. When you start getting cold, these thermostats signal the sweat glands, pores, and small blood vessels near the surface of the body to close down. Glandular activity and muscle tension cause you to shiver in order to produce more heat, and your senses send a very clear message to your brain, leading you to keep moving, to put on more clothes, to cuddle closer to someone, to seek shelter, or to build a fire.

Homestasis seems to be the rule when it comes to systems, yet we often forget about it, or think we’re not subject to a simple law of nature. But we needn’t totally despair. Homeostasis is often quite positive, and it keeps systems alive and well. Our bodies wouldn’t work without it, nor would our social systems.