I think the role of light in gothic architcture comes down to two differant factors. The first is that it came in when their engineering reached a level of sophistication that allowed them have larg expanses of glass on their wall. I am speaking from a simply structural point of veiw. Early on the walls of these larger buildings were very important for structural integrity . As their skill at building these larger structures increased and they came up with newer ideas for structural support such as the flying buttress, they simply had more options for adding in windows without the loss of structural stability. The other important thing to realize is that in letting in all this light it has to have made the interiors of the churchs much more warm and inviting. I have had the chance to visit variouse old churchs and cathedrals and I have to say I have always found those with fewr windows somewhat suffocating. Even those that were lavishly decorated still seem somewhat stifling to me. Also I feel that letting in all that light made the building feel less like a fortress.
Plus I think that at this point light was really getting linked with the whole idea trancendence, what with the halos popping up in the art and all. So basically people are really starting to expect the church being the house of good to have some of the divine light of heaven. After all if your gonna make me stand shoulder to shoulder with a bunch of other people (no deoderant!) for hours on end at least give me some light if not some fresh air.