When a light or another bright object is within the field of view of the lens (and not close to the edge) then the light rays from that object are close, or relatively close, to the optical axis of the lens. With the result being that most of those rays become image forming light rays.
Whereas with a bright object that is very close to the edge of the field of view of the lens a large portion of the light rays are not able to contribute to the formation of the image but are still striking the lens. Because of the higher angle of incidence, they produce a lot of reflections off of the surfaces of the lens elements, with the result becoming a wash of non-image forming light---->Flare.
Well, this sorta sounds good, doesn't it??