From howto:upload:

Uploading One's Own Art is Sacrilege

Posting one's own art is severely looked down upon, and there are two reasons behind this:

• Firstly, artists tend to be biased when it comes to judging the quality of their own work. What looks good to you may be regarded as mediocre to Danbooru moderators.
• Secondly, allowing posting of own work (self-posting) will cause a sudden spike of sub-par uploads as users mistake us for another Pixiv or Deviantart, far beyond the scope and capacity of Danbooru.