Agreed (with col). Finding out after the fact that just having been nice to a given person meant something to them is quite rewarding.
This is by no means a big deal at all, but I had enjoyed it:
I was coming home on the freeway, and just as I'd spotted a cop clocking people on an overpass where they often sit, a younger (20-ish) couple passed me in a van (presumeably en route to a wedding, judging by their dress). Either of us would've gotten a ticket, so I sped up to catch them, caught their attention, and pointed out the cop. Both of us slowed immediately and neither got tickets. Like I said, not a big deal, but it felt rewarding to save them from getting pulled over on their way to some event. Even if it may have been a stupid impulse to accelerate towards a speed trap...