In the first film, Laura realizes that Scott really is Santa Claus because he gave her (back) the thing she wanted most of all for Christmas that year—he returned Charlie to their home. (That he took him in the first place is beside the point.)
All of the rest of the elves are cheerful, so why is Bernard so uptight? Well, given his high position and the legacy status of Santa, he's likely gotten impatient with giving the same spiel over and over again, in addition to becoming more and more jaded every time another Santa goes missing, which would no doubt be worse if he thought of the Santa in question as a personal friend.
Fridge Horror: A gay man, or one who'd simply be happy single, wouldn't be able to stay as Santa Claus for more than a few years unless they entered into a marriage they didn't want.