How does Helena explain Demetrius' sudden shifting of affections from herself to Hermia in her monologue in Act 1, Scene 1?
She's a sad character, really, I think. Her actual explanation comes at the very end of the soliloquy, and, like just about everything else in this play to do with love, it focusses on the eye as the receiving window of the heart:
For ere Demetrius look'd on Hermia's eyne,
He hail'd down oaths that he was only mine;
And when this hail some heat from Hermia felt,
So he dissolv'd, and showers of oaths did melt.
What "heat" might Hermia have supplied Demetrius' oaths to melt them? Is it that Hermia made some sort of reciprocation to Demetrius? Or is it simply that Demetrius did love Helena until he then "looked on Hermia's eyne" ("eyne" is the plural of "eye") and then decided that Hermia was prettier? Who knows. But that seems to be what has happened.
Helena also comments that everyone thinks that she's as pretty as Hermia, other than Demetrius
Through Athens I am thought as fair as she.
But what of that? Demetrius thinks not so;
He will not know what all but he do know.
And as he errs, doting on Hermia's eyes,
So I, admiring of his qualities.
He "errs", she says, - makes an error. He's got it wrong. He should be in love with Helena, and he isn't. So on the one hand, she thinks it's a mistake. On the other, it's because Hermia's eye is in some way more powerful than hers.