On some podcast, sometime, I heard Tim Ferriss comment that when a person would ask him “Should I write a book?” the first response that came to mind was “Probably Not.” The reason was that it should be obvious whether or not you should write a book. Basically if it hurts too much to hold it in, then you should let it out and write it.
That’s what purpose is. It is necessity. It is the thing you must express for the well-being of your own soul. It is a burning desire, it must be released into fruition.