Yesod - The Foundation
Sexuality and love are the deepest human emotions and therefore, fixing these emotions fixes a person deeper than anything else. That's why in Kabbalah, the Sefirah of "Yesod" refers to the Brit. "Yesod" means "foundation". Guarding the Brit is the foundation of Judaism.
The positive commandments of the Torah are like a building. The negative commandments, what we don't do, what's underground, what people don't see - that's the foundation of the entire building. If the foundation is weak, then the whole building is in danger of falling.
No matter how much Torah and Mitzvos a person has, if one is not careful in guarding the foundation, then even a weak wind can knock down the tallest building. But if the foundation is strong, then the building will last no matter what it looks like above ground, and no matter how strong the winds blow.