1 You are not used to a certain angle
Maybe you are not familiar with the angle you are looking at yourself from. What happens is that you look yourself in the mirror and see yourself from the front view, but not the side angle. So whenever you see your side view, it may seem less attractive to you because you do not see it often.
2 You are looking at the wrong side
No human on this earth has a 100% symmetrical face. It is pretty much possible that your left side looks are better than the right one or vice-versa.
3 Due to the lighting conditions
Lighting conditions are crucial as if we are looking at ourselves in the mirror, and the lights are too dim or extraordinarily bright or not at the right focus, you might see your ugly reflection. (See Why do I look ugly in Sunlight?)
4 Your belief about good-looks does not match your features
It may be your perception towards your looks that you do not look charming, but in reality, you look good. You might be beautiful, but your facial features do not match your personal opinion, and you end up thinking that you look ugly.
5 Your side view is comparatively less charming
If we compare, the side looks with front looks, be it any human in this world, the frontal view is better than the side view. It may be the case that you have just looked at yourself in the mirror from the front and then looked yourself from the side; this made you think that you look ugly from the side angle. But maybe that's not true! (See Why we hate to see photos of ourselves?)