"in my headcanon, alpha!Dave has asked Rose to marry him every time he visits her, as an ironic running joke between the two of them. she politely declines every time. just before they went to meet their deaths at the hands of the Batterwitch, he asked her one last time - just to keep with tradition, and to hide how freaked out he was feeling. that time, she said yes."
Every time they are surrounded by witnesses, and every time she turns him down.