Remember the context of this particular monologue by Helena - it has just come after she has found out that Lysander is going to elope with Hermia, her friend. Helena is of course desperately in love with Demetrius and will do anything to gain his affection - even if it means turning in her friend to Demetrius. Her comments thus capture the ambivalence of love:
Things base and vile, holding no quantity,
Love can transpose to form and dignity.
Love looks not with the eyes, but with the mind,
And therefore is wing'd Cupid painted blind.
Nor hath Love's mind of any judgement taste:
Wings and no eyes, figure unheedy haste.
And therefore is Love said to be a child,
Because in choice he is so oft beguil'd.
This quote really strikes at the heart of what this play is all about. Helena paints a picture of love that cuts through the romantic twaddle that we so often associate with love. She starts of by saying that love has the magical ability to make us think that things "base and vile" are actually incredibly beautiful - so obviously seen when Titania falls in love with Bottom and his ass's head. Love does not look with the "eyes" (symbolising reason and logic) but with the "mind" (symbolising fantasy and emotions). Thus love is often "beguil'd" in its choice because logic and common sense have nothing to do with who we fall in love with and its effect on us. Helena is of course a prime example of this at this point in the play - she willingly betrays her best friend's secret in the hope that this will gain the love of Demetrius, even though he is very strident in protesting his hatred of her.
The rest of the play goes on to prove Helena correct as the Athenian lovers and Titania fall in and out of love with different characters. Love is shown to be an unstable force, which can be quite cruel and savage, as well as wonderful and something to be praised.
She loves Demetrius the good and the bad of him. It hurts her that Demetrius is going back and forth between her and Hermia.