Jake, who has been rendered impotent from a wound received in World War I, loves Brett, but can't consummate the relationship. Brett takes lovers, but Jake becomes particularly upset and jealous when she enters a sexual relationship with Robert Cohn. Jake doesn't think much of Cohn, and Brett's love affair with him is a particularly hard pill to swallow because Cohn didn't fight in the war. Ironically, in Jake's view, because Cohn has consistently been less of a man than Jake (not fighting, for example, and thereby not getting wounded) he is able to earn the prize of Brett.
For revenge, Jake helps Brett pair up with the young bullfighter Pedro Romero. Jake is jealous of Romero for a different and deeper set of reasons. Romero is not only physically potent, he is also emotionally whole, which gives him power. The "lost generation" alienation and loss of direction that tortures Jake is not part of Romero's makeup. He is still at one with his world, never questioning its values. That gives him a wholeness that is deeper than whether or not he suffers a physical injury.