Well the big reason comes down to shape. When it comes to your body, you have a definite shape. Some dresses will flatter certain shapes while other dresses will accentuate your flaws and make you look bad.
The same idea goes for your hairstyle. If your hairstyle is really designed for someone with a long face and your face shape is round, then that hairstyle will not be very flattering. The ticket is to find out what your face shape is and look for hairstyles that are designed for that shape.
Another great approach is to get several photos of hairstyles that you like and take them to your hairdresser at your next appointment. Ask you stylist for his or her opinion on whether the hairstyles you have selected will look good on you.
Don't forget to consider styling time when you are picking a new style. Some styles will require a lot of styling time in the morning.