$X makes a difference of F_me(X) in solving whatever problem it is we're caring about.

F_me(X) is valued by me at something less than $X, so I would prefer to unilaterally withhold my contribution.

However, SUM(F_k($X_k)) over everyone may still be valued by me more than $X, so I would prefer to give if everyone else giving is contingent upon it.

If we agree voluntarily to force ourselves to play by the rules, we can all benefit.