I'm only in the first couple of chapters and got introduced to the 'Upper Limit Problem,' which I find explains many of the limiting beliefs I've faced.
The basic idea is that when we achieve something great or have a feeling of contentment, there is something in us that looks for something bad in it. The reason behind this is that we are often in a state of not feeling like we deserve anything good, and so when good comes, our brains try to dampen it to a level of 'comfort,' which in this case would be looking for something wrong.
They gave a few examples of this in the professional setting, where people make great breakthroughs in their work, and suddenly it falls apart. One that caught my attention was that of a husband getting a huge promotion at work, coming home, and fighting with the wife.
We don't feel like we can have complete happiness and contentment, so we limit ourselves to a state where there is some comfort, but unconsciously train our minds to look for something wrong when too many things go right.
Two sentence description of the 'Upper Limit Problem':
Our brains are brought up with the idea of unworthiness. When we get something good, our minds go to something bad to run away from the idea that we can be happy and content.