a) remembering something delicious that you saw earlier and didn't have room on your plate for
b) wanting more of something you just tried
c) the desire to make the price you paid worthwhile
d) the need to illustrate to your friends that your stomach is indeed a bottomless pit.
"I disagree. In about 10 minutes my stomach is going to realize that it's full, so I think I'll just make use of the buffet momentum that I've developed and get dessert right now."