From what I got from the essay (it's been a while...), people who chose a "serious" path in life see no value in things that do not contribute to their goal.

E.g.: If you focus too much on engineering, you stop being able to see value in other aspect of life, such as philosophy.

De Beauvoir describes a couple of states of freedom of mind: 1. Children who believe in what they're told and don't think about their freedom 2. The serious man who chose a goal in life and do not think about their freedom (which the quote is about) 3. Nihilists who realize they're free but chose to do nothing 4. Adventurers who accept their freedom, but chose no precise goals 5. Passionate men who choose a goal according to their freedom and show no concerns for others 6. Genuine freedom (freedom + goal + concerns for others freedom)