Your beloved adult child is about to embark on a lifelong commitment about which they understand nothing. So it’s not surprising you’re as alarmed for them as you would be if they were sailing the Northwest Passage in flip-flops. Whichever phrase of warning or concern springs to your lips, however, hold it in. Your child needs support, not dread.
So if you can’t pretend you’re thrilled, find something supportive to say. It will be appreciated.