Impure pleasure leads people to sin. But in his epistle, James, the brother of Jesus Christ and the leader of the Jerusalem church, is not just talking about bad inclinations. Rather, he is speaking about those seemingly pure and good desires that spring from selfishness. For example, studying, working day and night, and seeking success and diligently pursuing a career are all good—but it is possible that someone wants these things for purely selfish motives.
Why do these inclinations and desires cause controversy and strife? It is because they come from selfishness. We seek them for our own gain. Our motivation for work, study, and effort is to benefit ourselves. We are not in building the kingdom of God or helping others or glorifying God. Our seemingly pure desires are only for our personal profit. This is selfishness, and any selfishness is sin.