Glare comes from stray light scattering inside the eye's own optics (cornea, lens, vitreous humour), which lays a veil of extra luminance over the retina and reduces image contrast — that's disability glare. Discomfort glare is milder: high brightness contrast that's simply annoying without impairing vision. On the hardware side, overdriven chips, cheap constant-voltage drivers, overly narrow beam angles, and degraded reflectors are the usual causes; constant-current drivers, proper TIR optics, adequate heatsinking, and physical baffles or louvers are the fix.