Why do all of the noble gases have electron affinities greater than zero?
All of the Noble Gases actually have electron affinities of less than or equal to 0.
This is because all of the Noble Gases have complete valence electron shells. They have a complete set of 8 electrons.
Think of 8 as being the best car that everyone wants. Most elements 'want' to have a complete electron shell with 8 electrons. Since the Noble Gases already have that 'perfect status' then they have an affinity of 0.
Affinity is the change in energy of the atom when an electron is added. Noble Gases are at the perfect number of 8 electrons. They don't 'want' anymore electrons, so there's zero change in the energy of the atom.
p.s. This is also why they are the least reactive elements on the table.