Think of Thetis like a stage mother... except the "stage" is the Trojan War and the "mother" is actually the freakin' goddess of the sea.
Thetis, a goddess of the sea, is the mother of Achilleus. Her husband—though they seem to be estranged—is the mortal Peleus. As a result of this mortal contamination, Achilleus does not inherit immortality from his mother. (For a discussion of the mythological connections between the marriage of Peleus and Thetis and the origins of the Trojan War, see our discussion of The Backstory's Backstory from the summary of Book 2.)
For Thetis (as we can easily understand), this is extremely troubling:
Thetis answered him then letting the tears fall: "Ah me,
my child, your birth was bitterness. Why did I raise you?
If only you could sit by your ships untroubled, not weeping,
since indeed your lifetime is to be short, of no length.
Now it has befallen that your life must be brief and bitter
beyond all men's. To a bad destiny I bore you in my chambers." (1.413-418)
Her character as we encounter her in the Iliad is both extremely eager to please Achilleus, yet tormented by the knowledge that everything she does to help him—like convincing Zeus to favor the Trojans, or getting Hephaistos to make him a new suit of armor—will only end up making his brief life even more miserable.