Quote 1
DEMETRIUS
Lysander, keep thy Hermia. I will none.
If e'er I loved her, all that love is gone.
My heart to her but as guest-wise sojourned,
And now to Helen is it home returned,
There to remain. (3.2.72-76)
Demetrius has been enchanted, which should excuse him, but remember that this will be his second transformation in love. First he loved Helena, then he loved Hermia, and now he loves Helena again. Though we know this last transformation was caused by magic, he was fickle even before being enchanted.
Quote 2
DEMETRIUS
You do impeach your modesty too much
To leave the city and commit yourself
Into the hands of one that loves you not,
To trust the opportunity of night
And the ill counsel of a desert place
With the rich worth of your virginity. (2.1.221-226)
Once again, we see love causing characters to act foolishly.