Now, look at the nearest flat surface. It looks perfectly flat, but it isn’t. If you zoom in, to a molecular level, you’d be able to see that the surface is really, really bumpy. Imagine playing ping-pong on a table that wasn’t flat but was instead covered in bumps and deformities. When the ball hits the side of a bump it wouldn’t reflect and keep moving to the other side of the table; it would probably be deflected to the left or the right or maybe even straight back to you. The exact same thing happens with light: when it strikes the surface that’s bumpy at the molecular level, it can bounce in pretty much any direction.
Most dry surfaces are pretty bumpy, but water likes to lie flat and isn’t as bumpy as those surfaces. When light strikes the water, it’s more-or-less deflected all in the same direction because now the surface is much smoother. This is why wet things are shiny: because the water is making the surface smooth (again, on a molecular level) which makes any light that strikes it reflect not randomly, but in the same direction.