The TL;DR version: Love makes fools of us all.
Lessons for Lovers (The Hávamál, verses 89-93)
Now plainly I speak, since both I have seen;
unfaithful is man to maid;
we speak them fairest when thoughts are falsest
and wile the wisest of hearts.
— Let him speak soft words and offer wealth
who longs for a woman’s love,
praise the shape of the shining maid —
he wins who thus doth woo.
— Never a whit should one blame another
whom love hath brought into bonds:
oft a witching form will fetch the wise
which holds not the heart of fools.
Never a whit should one blame another
for a folly which many befalls;
the might of love makes sons of men
into fools who once were wise.
The mind knows alone what is nearest the heart
and sees where the soul is turned:
no sickness seems to the wise so sore
as in nought to know content.
(Olive Bray, trans. )