In book five of "The Odyssey",why does Zeus,despite his liking of Odysseus,allow Poseidon to make this journey such an ordeal?

An interesting answer to this question is the free will vs. fate dilemma that is so present in Greek literature. In this case, we could argue that Zeus' actions represent a melding of the two sides. On one hand, Odysseus is fated to succeed by Zeus' favor. On the other, however, Poseidon's troubling power makes for a grand test of Odysseus' will to suffer and overcome.