It seems more than just a coincidence that Marianne and Eliza II are both 17 when they become involved with Willoughby.

IMO, from the moment Marianne becomes involved with Willoughby, she makes herself extremely vulnerable. She thinks Willoughby is the man of her dreams (or fantasies). She believes that lovers should agree with each other on everything. Her attitude about Col. Brandon following the cancellation of the Whitwell excursion reveals Willoughby's influence on her, as was previously discussed --> HERE

Marianne also seems to disregard propriety at times, engaging in activities that she should avoid (visiting Allenham). Her reaction when Elinor brings it to her attention seems to me like rationalization and justification. She seems ready to convince herself that there is nothing improper in her decisions. "Giving in" to feelings is what she is all about. Very dangerous, IMO, as was previously discussed during the GR --> HERE