Boredom Leads to Cynicism?

Okay. Throughout the first two chapters of The Sun Also Rises cynicism plays a large role in every character's attitude. Whether it be toward their career, their city, or their love life, they all announce the unhappiness they feel. For example, in chapter two, Robert's persistence in encouraging Jake to leave town with him leads him to expressing his dislike for Paris. "I'm sick of Paris, and I'm sick of the Quarter," (pg 19).
I'm just confused as to what is leading to the negativity. They live in one of the most visited cities in the world with some of the most beautiful people. What is there to complain about? So far, my theory is boredom. Maybe the characters are unhappy due to the routine their lives have taken. Guess we'll have to find out...