Being desired by Zeus over Hera elevates Thetis to an anomalous position above Zeus’s jealous consort, who claims to be “highest and best” of the goddesses. The same is true of paramours Demeter and Leto who produce glorious sons. All actually or potentially displace Hera in Zeus’s bed. For Thetis, whom Hera claims as a virtual daughter, both the breach of the parent-child boundary and the inversion of the parent-child hierarchy are particularly apparent.
Because Thetis has been humiliated and betrayed by a strategic marriage, her superiority as a favored daughter is called in question. She can re-create her sense of superiority—to the queen and foster mother to whom the father preferred her and to the husband whose bed he forces her to share—by rearing a son whose matchless superiority testifies to her own. She can accept the sacrifice of inferior marriage on condition that she be compensated with a glorious son.
Becoming Achilles, 29