It’s such a tragic moment because both their responses were reasonable.
T’Pring is about to be Spock’s wife, and it’s not simply a business union–she’s very much in love with him. She’s eager to be his partner and keeping changes in his life from her compromises that partnership.
The thing about Spock is that up to the end of the episode he’s still wrestling with the isolation that being bi-racial has come with–he’s aware that T’Pring should be let in but emotionally he’s never come around to that, having grown up at odds with other Vulcans.
What saddens me is that if Spock had communicated how his status affects his approach to full-blooded Vulcans (indeed if had even known to communicate it), I have no doubt T’Pring would have been much more forgiving… alas we sometimes figure out ourselves too late.