I can honestly think of a lot more than five reasons but for the sake of keeping this short and sweet, I'm listing the biggest in this entry.
1. Most people don't clean it thoroughly off the lashes which leads to build up and infection.
2. If you cry, sweat, swim or sit in a steam room, you can almost guarantee it will be running down your face giving you raccoon eyes.
3. It clumps! Eek! Mascara makes your natural lashes stick together so it's difficult to achieve a soft and fluffy look.
4. It's time consuming putting it on. The layering process in order to get a full look takes at least 15 minutes a day.
5. Taking it off is a nightmare which takes you right back to number 1.
Solution: Get lash extensions. :-)
