The Col seems to give him self away in the first para that you have quoted.

IMO he would like Marianne to think kindly about second attachments because he himself has had a first one? (We know about is unfortunate life and there is all this gossip about natural children etc)
Proceeding on this assumption:

I feel here he sees in Marianne as he himself was recently.
It seems he hasn't had a second attachment, perhaps not believing in it himself, till now.
His utterance makes sense;
"This," said he, "cannot hold; (it didn't for him)

But he wouldn't want the romantic sentiments she has to change in order to enable her to believe in second attachments. It shouldn't be the result of necessity, cynicism etc. but something rising from romantic thoughts as before.

And that's what I think attracts him towards her.
She is someone who would fulfill his above wish, who would bring back his belief in something he seems to have lost.

I would say he is showing some kind of 'sensibility' here, a character he shares with Marianne but in a better way.