What do you think is the reason of fading of tattoo eyebrows?

My friend had the tattoo on her eyebrow one year ago. She was very happy with the result and she enjoyed her decision. Later on she noticed that her eyebrows are getting fade. This made her revisit the dermatologist again every week. I want to know does this happen in every case? One has to revisit the dermatologist again and again once the tattoos for eyebrow start fading? What are your views about this? What happens when you go for retouch of your faded eyebrows?