Why shouldn't I master my music myself?
Nowadays technically it would be possible to master your tracks yourself. However you should carefully consider if this is what you want to do. To master your tracks properly you need an acoustically optimized studio and you must have a monitor system that does not have any coloration of the sound.
Another component that you should take into consideration is that during the mastering you must be able to listen to your music objectively. Mastering decisions must be made without thinking about the producing process. By this I mean you can’t be mastering a track when your also taking into account the hardship you had getting that one recording right. It can also be tempting to overdo the mastering by giving your track a little bit extra bass compared to other tracks. This will only overdo your track and will rather damage the sound than making it better. It might sound great in your own studio but it might not result in a good sound in the clubs or on the radio at all. Mastering requires a different type of listening, comparing different sounds and making small but important adjustments.
To summarize the above it is not impossible to master your own music but it can be extremely difficult. Without any experience you’re bound to get a disappointing result. Having spent so much time and energy in creating your track this last but critical stage deserves the same attention as it did producing it.