WHY DOES THIS HAPPEN?
There are a number of different reasons why fading of the leaves on your plant can occur. For instance, it could be a sign that something's wrong with the environment around your plant, such as it getting too much light. Some plants are simply more sensitive than others, and excessive amounts of harsh, direct sunlight can sometimes be damaging, particularly for variegated plants.
Additionally, certain plants are particularly sensitive to lime deposits found in hard water, and this sensitivity can exhibit through discoloration in the leaves. Or, it could be a sign of a watering imbalance - both overwatering and underwatering can cause leaves to fade in color, as can overfertilizing.
A condition called chlorosis commonly causes yellowing of leaves as a consequence of a lack of chlorophyll, typically caused by excessively high soil pH and / or compaction, as compaction of the soil can, in turn, result in poor drainage. If you suspect your plant's soil has become compacted, this is a sign that it's time to repot it! Finally, fading of the leaves may be due to a Spider Mite infestation. These are small pests that can quickly take over your plants if left untreated, so be sure to thoroughly inspect both the undersides of the leaves as well as the leaf surface to check that your plant doesn't have a pest problem.