Why does Zeus deny Achilles's prayer for the safe return of Patroclus?

In Book 16 of the Iliad, Achilles prays that two things will return safely from battle: his dear friend Patroclus and the Greek ships. Zeus decides he will only grant one of these requests. As the battle rages, Zeus makes his decision. After Patroclus kills Zeus's son Sarpedon (Zeus could have saved Sarpedon, but the other gods say that would set a bad precedent), Zeus decides to kill Patroclus in retaliation. However, he first lets Patroclus win a resounding battlefield victory. After that point, Apollo wounds Patroclus, and then Hector kills him.
Achilles is devastated at the loss of Patroclus. Patroclus's death, however, becomes the catalyst for Achilles to do what he said he would never do: rejoin the fight against the Trojans.