The mirror can be your friend or your enemy. Bodybuilders use the mirror to gauge their progress. Although sometimes accused of being narcissistic because of the time spent looking at their bodies in the mirror, bodybuilders actually use the mirror to simply monitor their progress. (The man on the street is often not aware of the subtleties and details of the physique which the bodybuilder is checking for.)
The mirror is also a very helpful tool during a workout to check form and keep oneself safe and on track. There are a number or exercises which I perform better and with more proper form if I watch myself in the mirror.
(While at the gym, always take note of where you are standing in relation to your fellow gym members. Someone may need to see themselves in the mirror as they are exercising, so pay attention and move out of the way if you’re blocking someone’s view.)
As this cartoon illustrates, we don’t always see what we are actually looking at in the mirror. It’s common for the mind to play tricks on us. (This cartoon plays on common gender stereotypes, but I can attest that men can have the same dismorphic view of themselves in the mirror as women!)
Some days I look in the mirror and see a huge bodybuilder, while other days I see the skinny kid I was in 7th grade. This is why I suggest you take photographs so monitor your physique. Looking at yourself in a photo is often the best way to “step outside” your body and get a realistic view of your progress.
Also, let me warn you that not all mirrors actually reflect back a perfect image of yourself. I have been in plenty of gyms where mirrors are actually warped or bent, or have even been installed at an odd angle, giving you a slight “fun-house” reflection.  Also always take into account the effect of lighting on the image in the mirror. Be careful to monitor this situation so that you don’t overact to what you think you see.
Let the mirror be your friend, and use it to help with your workouts. But be leery of giving the mirror extra importance or power. It’s just a tool and can sometimes fool you. Let it help you keep those barbell curls in strict form, then move on knowing you have done all you can to improve your health and physique.