imagine Lady Elliot went through somewhat of the opposite of Anne's experience. Anne had every side against the marriage - especially Lady Russell warning her to be prudent.

It could be that Lady Elliot, at a relatively young age and relatively inexperienced in matters of love, had everyone pushing her towards marriage.

He was handsome, rich and titled. She would be well settled and cared for. He seemed to be attached to her - she was able to make him economise. Though with friends like that...

Sometimes you stumble into something and get wrapped up in what others want for without stopping to think about what you really want. I imagine no one ever stopped to talk rationally to this young lady in love. No one told her to be prudent or serious. They merely encouraged her and hoped for her success.