Part 6 is a warning against taking pride in your posessions:
Be not elated at any advantage which belongs to another. If a horse when he is elated should say "I am beatiful" one might endure it. But when you are elated, and say "I have a beautiful horse", you must know that you are elated at having a good horse. What then is your own? The use of appearances. Consequenty when in the use of appearances you are conformable to nature, then be elated, for then you will be elated at something good which is your own.
Although the language is somewhat convoluted, the intent is clear: don't base your pride and self-worth on things which are not inherently yours. If you act in a way that is "good" and naturally yours, then it is proper to feel joy.
Relating this back to the dichotomy of control, why would it be wrong to feel pride for something that you do not fully control? Because you open yourself up to manipulation and disappointment. If your self esteem is based on your posessions, then it is fragile, because your posessions can and will disappear over time. If, however, you value yourself based on the righteousness of your actions and thoughts, nothing can make you think less of yourself.