Mastering Your Own Music
This wont really be about why you should avoid mastering your own music. It is not a bad thing to attempt to do if you don't consider yourself a professional musician that is only making music for a hobby.
But if the, training, equipment, and studio is there, I guess it is possible. However there is some good points as to why mastering music on your own may not be a good idea.
Most likely being inexperienced in the field is going to mean a lot of mistakes producing less than professional results.
You might achieve less than 100% of excellence because lack the knowledge of certain areas in mastering.
You also will hear your music differently from the mastering engineer and that may persuade you to want to do things to the song that wont fare well in the mastering process. Of course you can work with an engineer about your ideas so that they can be implemented the correct way.
Your environment can an most likely will be wrong for the listening process and for that matter, a lot of things might be done to a song that shouldn't or is not even needed.
So I'd highly advise that if there is doubt in what you are doing when it comes to mastering your own music, seek out the pros an put them to the test.