[Rain Man]

It's supply and demand. When the supply of pizza slices is high, they have less value. When the supply is low, they have more value. If there's only one piece left, then it ostensibly has a very high value, which makes the decision of who gets it a weighty one.

Now, to some extent, this is offset by changes in demand. When there are a lot of pieces of pizza, demand is also presumably high because people are hungry. As the supply goes down, so does demand. Therefore, there's an economic self-regulation that takes place.

However, that regulation is not perfectly proportional. For example, if you have two anorexics and 400 pieces of pizza, demand goes down faster than supply, and therefore the value of each piece goes down faster than the supply, so that the "last piece" really has no value (in part because there is no last piece).

On the other hand, if you have two professional wrestlers and only five slices of pizza, the supply goes down much faster than the demand, and the value of each slice goes up dramatically as supply goes down. The question of the last piece then becomes whether it is worth more or less than the medical cost that results from being slammed in the head with a folding chair.

All in all, it's a very dynamic and complex process.