Helping people out is pleasing. Quite often there'll be people standing beside the train confused by the front 4/rear 4 split, or confused as to which tube line to get, or which trains gets them somewhere quickest, where locations/streets/attractions are in central London, etc. You can tell most people are genuinely grateful and appreciate the help, that goes a long way to making me smile. I recently helped an elderly gentleman as he tripped between the gap between the train and the platform - he burst out crying and was ever so grateful. It made my day.
Other times is when I overhear young kids talking about trains or asking questions to the people they're with. I remember one boy perpetually asking his mother about where each train was going that he saw - she didn't have a clue, but I was able to tell him each time and I could just see the excitement on his face. Another kid recently wanted a ride on a 395 but nobody knew which service would be the next one through, again telling them he got all excited and it made me smile.
Watching the sunrise can be quite pleasurable too along the north Kent coast. During the winter months I was able to watch sunrise and sunset each day from the train, was lovely.
I've heard a few funny stories from fitters too. Some were based at Gillingham and had been called to a problem with a train. They'd been told that the unit was at Stone Crossing, so they got in their van and headed off to the station near Dartford, called Stone Crossing. They arrived and there was no train to be found... only for them to ring up and realise the problem unit was at Stone Crossing (a level crossing, not the station) near Faversham, a total distance of about 40 miles away. Funny now and something to smile at, but at the time...