Because he walks barefoot through the water at the beach with his date? . . .
Seriously though . . . She/he hasn't been turned off by the man at this point, so the man starts wondering if it could be serious and what to do if it goes further. Will it become a relationship? Will he have to meet the parents? How many kids will there be anyway?
A sense of panic can set in and the tootsies get a little chilly. :)