Adding highlights or lowlights can be a refreshing addition to your favorite style. From time to time we all want a change but it's hardest to decide what that change should be when we feel like we've finally found our favorite style and color.
The great thing about highlights and lowlights is that they can be as subtle or dramatic as you want and either way it's going to feel like a great change. They can lighten or darken your color all over if you're transitioning into a dramatically different shade, they always add dimension to your color, they can draw attention to and brighten your face, or accentuate your cut or the texture of your hair.
If you're pretty happy with what you've been doing with your hair but are still dying to try something new they're the first thing to consider.