Say you have 365 people living on an imaginary island in an imaginary society. Every day all 365 go to the river to catch fish with their hands. They eek out a meager living this way with each person catching 1 fish each day. One day someone gets the idea of creating a net. he thinks that if he makes this net he may be able to catch twice as many fish per day. In order to create the net he would have to go a day with out fish, a very large cost indeed for a person who is already on the brink of starvation. Under socialism he would get to use his net 1 day per year. it is very unlikely that he would decide to go a full day with out eating in order to have a net for 1 day per year. In fact everyone who ever thought of the idea would come to the same conclusion and no nets would ever be made and the society would net (no pun intended) 365 fish per day. Under capitalism anyone who made a net would get to use it every day, and so everyone would decide it was worth it, everyone would make a net and the society as a whole would net 720 fish per day. This simple change of allowing the person to keep the products of his labor for himself has made the society as a whole twice as wealthy.

obviously i oversimplified and took a lot of things for granted. for example its unlikely that EVERYONE would make a net even under capitalism but it ought to be sufficient to communicate the general idea of dispersed benefits and concentrated costs (its weird saying that because im used to talking about the problem of concentrated benefits and dispersed costs).