Something happened: they had a "home date" with a preview of how it would be to live together, and they got along just fine.
Times like that, when you have lots of times to talk about things and hang out and see if you'll fight instead of get along are very important in a relationship. There comes a time in dating where you can't always put your best food forward all the time or always be wearing a facade.
The sooner people get "real" with each other, the sooner they find out if their relationship is actually real.