I noticed in recent times that there is a interesting little phenomenon that happens at cocktail parties, receptions, or anywhere with hors d'oeuvres on trays. Actually, there are two interesting things that happen:
1) No one ever wants to take the last of something.
2) Depending on the crowd, people are sometimes hesitant to take the first one.
Why is this? Well, for #1 you may say that it's cold, or not fresh, or whatever, but I'm going to go deeper. I propose (with no evidence!) that it is an evolutionary trait to not eat the last of something. The last of something probably spoiled in the bad ol' days, so there was an evolutionary pressure against those brave souls that ate the last of those mammoth canapes. It is probably simpler, that people don't want to look like gluttons, but really - gluttons? Americans don't want to look like gluttons? Too late.
I do think that is what keeps people from diving into those spanikopita being passed early in the evening. They don't want to look like some food-crazed maniac. Of course, if you get a bunch of people who are admittedly so, then you start to feel bad for the servers. At a cocktail party last year servers didn't get more than two feet from the kitchen before an army of old, rich, foodies practically gummed them to death.
So what is the solution? If you want some, get some. Don't worry about decorum and get yourself that last crab cake.