There is something about how you drew this eye that makes it look more like a cartoon drawing and not realistic. The eyebrow looks scribbled in, not well worked on. Look at this eye and you can see the difference. You shoudl draw more eyes and then see how they come out, I think it might be the pupil. Keep trying but I don't think you put enough effort into this. I try going on youtube and seeing how they draw realitic eyes, it actually helped me improve a lot better than before. You should try it, I am sure it will work. [link]