As you age, your lips naturally decrease in size. This is because the collagen that used to plump up your lips naturally decreases. Additionally, spending a lot of time in the sun, smoking or developing a large number of wrinkles can also decrease the collagen normally found in your lips. If your lips are thin or have decreased in size, you may be looking at using fillers to plump them back up. Here are a few reasons why fillers for your lips may be right for you.