Why are fats glossy?
Reflected light (gloss) is a maximum when the angle of observation is equal to the angle of incidence:  https://en.wikipedia.org/wiki/Gloss_(optics). The image shows reflection from a plane surface with some roughness that gives diffuse reflection.



At Brewster's angle, the reflected ray is strongly polarized. The image is intended to indicate a depth of penetration in transparent materials; the polarization is not relevant here.



In conductors, like metals, and strong absorbers, like iodine, there is essentially no transmission, and almost all the light is reflected from a plane surface, because the penetration depth is exceedingly small.

In a dielectric (i.e., a nonconductor like glass or fats), some light will be transmitted, and reflection occurs after a penetration of some depth - on the order of a wavelength of the light. Glass is atomically uniform on this scale, and  a fat molecule (e.g., glyceryl trioleate) would be a glob about 2 or 3 millimicrons in diameter, therefore essentially homogeneous on this scale. Any liquid volume larger than that would have no discernible inhomogeneity and reflection would be a uniform wavefront, specular (mirror-like) rather than diffuse. 

But suspensions and runny emulsions and milk are not homogeneous on that scale. They have particles on the order of a wavelength of light (~500 millimicrons) in the liquid, so reflection occurs from sites of different composition within this penetration depth. These materials reflect similar to reflection from a roughened metal surface and give a diffuse reflection rather than a gloss.