Lightening is the process of making something more light (to make it weigh less, or make something brighter, comes from verb 'to lighten'). Lightning is what comes from the sky before thunder.
In this case, I don't think either are correct. You are probably looking for "enlightening" which is similar to lightening but is used in more of a philosophical sense.
"The enlightening of happiness has told me that happiness is the meaning of life" or something like that. "Enlightenment" sounds better to me in this case.