Q: How does light bouncing off an object affect how we see it?
A: It's technical, so in lieu of writing a long response, here's a short and sweet video on how light affects what we see :)
So why are trilobal threads so shiny? Part of the reason for this is the initial shape of each fiber. The final thread strand is a product of many micro strands. Each micro of a trilobal fiber is 3 sided (trilobal means 'tri = three 'lobal' = sides.). More light reflects back to our eyes when looking at a trilobal thread than a round or irregular thread.
Most filament polyesters are round and most viscose rayon threads are irregular shaped. However, when the micro strands are tightly wound together and processed, the end result gives us a seemingly smooth, even thread. Just like the cat hair example in the video, when looking at the big picture, it's soft and smooth.