When Brynhild meets Sigurd, she passes her vast wisdom on to this strange man without hesitation. She shares not just her common sense, but also everything she knows about runes and magic. He's an appreciative audience -- an admiring audience even, appearing to take it all in with the respect and weight it deserves.
Specifically, Brynhild tells Sigurd:
Ale runes shall you know
If you desire no other's wife
To deceive you in troth, if you trust.
They shall be cut on the horn
And on the hand's back
And mark the need rune on your nail.
For the cup shall you make a sign
And be wary of misfortune
And throw leek into the liquor,
Then, I know that,
you will never get
A potion blended with poison.
Not much later, Sigurd begs Brynhild to tell him more of her wisdom, and she says this:
Beware of ill dealings, both of a maid's love and a man's wife; ill often arises from these.
And do not swear a false oath, because hard vengeance follows the breaking of truce.and then:
Beware of the wiles of friends. I see only a little of your future life, yet it would be better if the hate of your in-laws did not descend upon you.Sigurd professes his admiration of Brynhild's wisdom, and we are left with the impression that he takes her words to heart, treasuring them along with the apparent love he holds for her after this first meeting. Brynhild accepts Sigurd's praise, and admiration, but she warns him that they are not meant to marry or live together. Still, Sigurd insists, and Brynhild is persuaded. They exchange vows with one another, privately, and then part.
In these three pages, Brynhild has both warned Sigurd of how he will be betrayed and given him the information he needs in order to prevent his downfall. Everything that happens to him after this point might have been avoided, had he used the "magic" Brynhild gave him, and kept her words of warning and wisdom in mind. Had he used the runes, Queen Grimhild could not have used the ale of forgetfulness on Sigurd, which caused him to forget his love for Brynhild. Had he been cautious of other women, and the "wiles of friends" he would not have trusted her to drink from the cup she gave him, and never would have married Gudrun, or helped Gunnar, his brother-in-law, win Brynhild's hand in marriage.
Brynhild knew everything that was coming, her own fate and Sigurd's, but even her knowledge and wisdom, given freely, wasn't enough to prevent the future from following that grim script. Because for all her wisdom and strength, she did not have the power to make Sigurd follow her advice.
Even for the wisest among us, wisdom will only get a woman so far.