Wow, I haven't seen anyone bring this up in concrete terms before, although I've heard Witnesses imply that they felt good after a day of service. Yeah, I definitely got that high -- I think I still do sometimes (I haven't been going out very often so it's hard to say). As others have said, I once thought it was a reward from Jehovah for doing the right thing.
Now I think I agree with TS that it was a feeling of sheer relief. Service has actually become harder for me as I've gotten older, not easier. The close-quarters social situation in the car and field service group is tough for me, perhaps moreso than talking to strangers at their doors. So getting to breathe freely again when I get home feels great.
Interestingly, I noticed when I was younger and would occasionally pioneer that I would regularly get this feeling after a morning spent in service, but the longer service days would drain me to the point where some time after lunch, I would start getting cranky, and when I got home I felt only tiredness, no euphoria. There's probably some science to be done on this phenomenon (maybe it already has).