I think that it really has to do with being able to express yourself as freely as possible. One of the key things is that when you write for yourself, it gives you freedom in what you write and what style that you do it in. But when you write for others, your voice may not be the one that gets published in the end. So it is really about control over your content and how you present it to the world.
I have recently started to work another small side gig, writing posts for a website. My main job is to write little blurs about the product and how it will improve lives and the human experience. The key thing to take from this is that it is a different style that I am used to writing, it is not about creating meaning or figuring out a problem, but to merely sell a product. I am having difficultly feeling authentic to the process and that is showing up in my writing.
I am sure that I will get used to it and learn to eventually get over this and be able to sell a product by using my words. It, like everything else is a skill that can be learned and acquired overtime. Although, now that I have started on this decadent journey of commercializing myself for money, I have a new appreciation for the many people out there with English degrees that are forced to write something that they may not have a passion for, but need the money to earn a living.
I want to help my friend's website and get his business up from the ground. By writing for him, I can grow my portfolio of work as well as learning to write when, I need to produce something in a timely fashion. It is not ideal, but neither is life.