為什麼有些物體會反光 有些不會?

其實所有物體都會反光（不然就看不見它啦）
不過有些物體表面較平滑（如鏡）就能將物體反射出來的光反射進我們眼中
而你所指的反光，則是其表面平滑度不及鏡，但足夠將光反射，這就是反光了

Reflection occurs. The angle of incidence as same as the angle of reflection. In a calm water surface, regular reflection occurs in the surface when light rays enters the surface. Reflected light will reflect to your eyes. This case will also occurs in glass and mirror too.
Other reflection is called diffuse reflection, the surface of the material like highway road is not a plane. No reflected light will reflect because the angles of reflection are different.