My thouhts on this matter:
Jane, we already know, has very strong convictions about her beliefs, whether religious or otherwise. It seems to me that she believed in true love and the notion that, if all goes well, everyone will find the person he or she is meant to be with. She's a romantic- this explains her initial refusal to an arranged marriage. So, this comment upsets her for a few reasons: first, she is not with the person she is supposed to be with (or so she thinks); she is with a sarcastic, offensive drunkard, who shows no affection towards her. And she's stuck with him- of course she's upset by her conversation with him. Also, Guilford is telling her that one of her beliefs is wrong, that love isn't as wonderful as she thinks it is, and that she is naive to believe in it. This also offends her
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I hope this analysis is helpful- it's just one idea that I came up with and was more or less satisfied with. I also have had trouble figuring out this line. Any other ideas?

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