Why is Zeus reluctant to grant thetis' request?

'This is a sorry business, indeed, and you will force a quarrel with Hera. She will taunt and rile me. As it is, she scolds me endlessly before the other gods, claiming I aid the Trojans in battle. Go now, before she notices, while I think the matter through. Come, I will nod my head, to reassure you, since you immortals know this as my sure pledge; once I give the nod, my Word can never be recalled, it proves true and is fulfilled.' - Zeus, the Illiad, granting the request of Thetis to honor her son Achilles; and explaining that in doing so he earns the ire of his sister and wife Hera.