1. You cannot please everyone. No matter how good of an artist, writer, designer, etc. you are, there will always be people out there that will not like it. You could spend hours, days, months, and put your heart and time into a piece and some people will still always find something wrong with it or just flat out think it is ugly or unappealing.
2. Critics are everywhere. Like number one, even if you were given an award for best artist ever there will still be people out there that will criticize your work.
3. Not all criticism will be constructive. You will find a lot of critics simply trying to help you improve, but some individuals will straight out say they don't like it or point out why it is bad, and there is nothing you can do to change their minds.
4. Your art won't always sell for money. A mass of people can greatly enjoy your work and appreciate it, but not everyone is willing to spend a large sum of money on an art piece. People who make clothing or costumes don't really need to worry too much about this since there are many more people that are willing to pay 200 dollars on a beautiful custom dress than 100 dollars on a painting or drawing to hang on their wall. Same could go for sculptors, but again depending on your work it can be very hard to sell your projects let alone try and make a living off it.
5. There will ALWAYS be someone better than you. No matter what you do, or what your profession is, or how original you try to be, there will always always always be someone out there better than you.