She had appealed to his most powerful passion, the hatred of tyranny.

She had excited his admiration by setting conventions at defiance, and showing her readiness to be his mistress.

Her confidence called forth his gratitude.

Her choice of him for a protector flattered him: and, moreover, she had acted on his advice to carry resistance a outrance.

There are many good Shelleyan reasons why he should elope with Harriet; but among them all I do not find that spontaneous and unsophisticated feeling, which is the substance of enduring love.
In the same series of letters, so incoherently jumbled together by Hogg's carelessness or caprice, Shelley more than once expresses the utmost horror of matrimony.