Today I ran some errands during my lunch break, and on a whim I picked up some carrot cake and some pastries. When I got back to the office, I sent around an e-mail saying I'd put some goodies out on the table near the 'fridges.
I checked the table about two hours later: it was like a plague of locusts had been through. I'm guesstimating that 20-30 people had some of the food -- which is fine. But: only one person thought to e-mail me to say "thanks".
The lesson learned:
Don't do nice things because you're expecting gratitude: you'll be disappointed. Do nice things because it's good to do nice things.
Which I'm fine with. But it's an important distinction.