Lydgate and Rosamond's Engagement

I hope I'm doing this correctly... Anyway, I was really interested by the haste I felt was employed to Rosamond and Lydgate's engagement - a haste that is pretty uncommon in a book as long as Middlemarch. While one could make the argument that the engagement was, in actuality, somewhat prolonged, considering the reader finds out about Lydgate's admiration of Rosamond in Chapter 11, in the course of Chapter 31, Lydgate completely changes his mind as to his feelings about Rosamond and marriage.

The Chapter starts off with a conversation between Mrs. Plymdale and Mrs. Bulstrode, who are speaking about the supposed engagement between Rosamond and Lydgate (which was, at the time, completely false). After Mrs. Bulstrode speaks to Rosamond and ascertains that Rosamond has ignored all other suitors for Lydgate while she was still unsure of his intentions, Mrs. Bulstrode immediately sets out to set Lydgate straight.

While Lydgate avoids Rosamond for a time, the first time he sees her again, he is suddenly overcome by the realization that he loves her and walks away from the encounter engaged to Rosamond.

What changed? What happened to cause Lydgate to be open to marriage, which he had completely denounced just a few pages earlier? I don't know that there is an answer, but I think it's an example of how Eliot doesn't really go into the details of how the happy couples of the novel are made, such as Celia and Sir John. Instead, Eliot is concerned with the undoing of couples, such as Dorothea and Casaubon.

Lydgate and Rosamond's engagement, to Eliot, would then be nothing more than a necessary plot twist. While Eliot elaborately describes their courtship so that the reader understands that the intentions towards marriage are one-sided, when the courtship comes to fruition, Eliot rushes to get the plot point out.

Indeed, Eliot only says, "In half an hour he left the house an engaged man, whose soul was not his own, but the woman's to whom he had bound himself" (pg. 190). However, in the same chapter (a strikingly small chapter), Eliot writes that Mrs. Bulstrode asked Mr. Bulstrode "to find out in conversation with Mr Lydgate whether he had any intention of marrying soon. The result was a decided negative" (pg. 187).

The reader sees no true change of heart in Lydgate, but is just supposed to accept the fact that one moment Lydgate has no intentions of marrying and the next is engaged to Rosamond. My guess is that to Eliot it doesn't matter, because it is the less interesting plot point than the sympathy unrequited love can invoke.