If the set-point theory is true why do people get over and under it?

1 Because body fat isn’t your only set point.

You have a body fat “set point.” But Paul Jaminet also hypothesizes that your body has an even more important set point for maintaining the health of your lean tissue. If your body isn’t getting the micronutrients it needs, it will try to get more nutrients using the same mechanisms that it uses when you fall too far below your body fat set point: increasing your appetite and extracting more energy from your food. If you’re eating nutrient-poor processed foods all the time, you’ll just stay hungry, because your body is desperately looking for nutrients by driving you to eat more food.

2 Because you eat more than your body needs. Pleasurable food overwhelms your body’s natural message of “OK, I’ve had enough now,” so if you don't control your self it is very easy to eat more than the body needs. Stimulating food is also easy to be addicted to. 

3 Sometimes insulin stays elevated all the time. This prevents you from running off your stored energy reserves, because you’re constantly in “storage mode” and never switch over to burning those stored calories. In this situation, you’re eating enough calories, but they’re not available for energy, so your body is starving (and you still feel hungry) even though you’re gaining fat. It’s the worst of both worlds. 

All kinds of things affect insulin levels. Just to name a few: sleep deprivation, chronic stress, exposure to environmental toxins, menopause, genetic factors, vitamin deficiencies, and the composition of your gut flora. It’s true that eating more carbs than your body can handle is one factor affecting insulin levels, but it’s far from the only problem! 

Regardless of how it starts, though, chronically high insulin can overwhelm the body’s “set point” and cause weight gain. Problems with insulin also affect another hormone called leptin, which regulates appetite and metabolism. The ultimate result is that your body is now “defending” a higher weight, making it very difficult to get (or stay) lean. 

What to do to maintain a healthy weight:

Eat nutrient-dense food.

1 Avoid food that is hard to stop eating or drinking. (Things with honey, syrup, sugar, chips, beer, wine etc.)

2 Avoid sweeteners that make you hungry. (I get hungry from Pepsi max (aspartam).

3 Make sure you have healthy levels of hormones like insulin and leptin so the calories gets burned for energy, not stored as fat by eating low-carb or carbs in moderation. (Read details in the article below.)

4 Avoid chronic inflammation because it elevates hunger and impairs carbohydrate metabolism. ( A recipe for overeating and storing those calories as fats.) Reduce inflammation by getting enough sleep, managing your stress, avoid extreme and punishing exercise, make sure you recover properly from your workouts, limit nuts and seeds, and eat plenty of fish (omega 3).

5 Intermittent fasting.  Intermittent fasting mimics the benefits of carbohydrate restriction: it lowers insulin and raises the levels of several other fat-burning hormones like growth hormone and adrenalin, prompting your metabolism to use stored body fat for fuel and lowers you calorie intake.

6 Exercise

7 Be careful with the consumtion of nuts, seed butters, dried fruits and fruit juices. 

All except the mentioning of alcohol is taken from the article below.

http://paleoleap.com/weight-loss-on-paleo-diet/
I continue adding more interesting stuff about addiction to stimulating food.
Our brains are wired to respond to the stimuli with which they evolved. For example, our natural taste preferences tell us that fruit is good. But what happens when we concentrate that sugar tenfold? We get a superstimulus. Our brains are not designed to process that amount of stimulation constructively, and it often leads to a loss of control over the will, or addiction. 

It's a very similar process to drug addiction. Addictive drugs are able to plug directly into the brain's pleasure centers, stimulating them beyond their usual bounds. Food superstimuli do this less directly, by working through the body's taste reward pathways. In fact, sweet liquids are so addictive, rats prefer them to intravenous cocaine. You can't take just one hit of crack, and you can't have just one Hershey's kiss.

This chapter was taken from the article below.
http://wholehealthsource.blogspot.no/2008/03/superstimuli.html#uds-search-results

A Curious Finding

It all started with one little sentence buried in a paper about obese rats. I was reading about how rats become obese when they're given chocolate Ensure, the "meal replacement drink", when I came across this:

...neither [obesity-prone] nor [obesity-resistant] rats will overeat on either vanilla- or strawberry-flavored Ensure.

The only meaningful difference between chocolate, vanilla and strawberry Ensure is the flavor, yet rats eating the chocolate variety overate, rapidly gained fat and became metabolically ill, while rats eating the other flavors didn't (1). 

Experiments in rats and humans have outlined some of the qualities of food that are inherently rewarding:

• Fat
• Starch
• Sugar
• Salt
• Meatiness (glutamate)
• The absence of bitterness
• Certain textures (e.g., soft or liquid calories, crunchy foods)
• Certain aromas (e.g., esters found in many fruits)
• Calorie density ("heavy" food)

We are generally born liking the qualities listed above, and aromas and flavors that are associated with these qualities become rewarding over time. For example, beer tastes terrible the first time you drink it because it's bitter, but after you drink it a few times and your brain catches wind that there are calories and a drug in there, it often begins tasting good. The same applies to many vegetables. Children are generally not fond of vegetables, but if you serve them spinach smothered in butter enough times, they'll learn to like it by the time they're adults.

Industrially processed food, which has been professionally crafted to maximize its rewarding properties, is a superstimulus that exceeds the brain's normal operating parameters, leading to an increase in body fatness and other negative consequences.

http://wholehealthsource.blogspot.no/2011/04/food-reward-dominant-factor-in-obesity.html
Here is a link to a article about leptin:http://paleoleap.com/managing-leptin-levels/