When I'm in the zone for building, debugging and testing a new system, it's pretty unlikely that I'll be smiling. Most of the time my thoughts are filled all the mistakes I have already made or are attempting to avoid making in new code. At the end of the process there's a pretty decent rush of accomplishment, but that's usually replaced by worrying about the next problem on the giant list of todos.

That said, one of last few places I worked at had a social hour every Friday at 4pm. I think that was a very good addition to the workplace that is well worth having to enable more casual team bonding on a regular basis. It serves a similar purpose as smiling at your co-workers, but is perhaps a bit less forced for those of us who have the pessimist / realist lookout on software development.