Sometimes we are all too eager to do things ourselves and try to save money doing things we may not be best at. Problem with trying to do things yourself is that you don't know the inside scoop on a lot of things. What type of roller to use, how to mud a wall properly, where to buy good quality paint for a good price. In the end, you are left with a job that you may not be that happy with and that has cost you more money in supplies than it should have and a lot of your time.
We recently did a quote for someone and they decided to paint their house on their own. They asked us to come by and just paint the stairway for them and they told us the paint they were using. This paint was just from a box store and was more than double what we pay at our paint stores for high-end products. This customer just spent an extra $300-400 more on product alone.
We don't discourage you from wanting to do it yourself, but when you think of what is really involved, it may be beneficial to have someone professional do it for you.
There are other times when I have heard someone say, "Oh, I decided to paint the room myself. It only took me a few days." So much time wasted where we could have had the job done in a matter of hours.