It could be because first, if I win, it's "Well, he's a pro. He should win. What's he doing here in the first place?"
Second, if I don't win, then it's, "He's a pro, but not a very good one. What's he doing here in the first place?"
But the real reason that I don't enter competitions is because, when I look at the winners, I realize that I've deleted everything that has a hope of winning? 
For instance, look at the winners in a local gallery here,
specifically the 2nd image ""Bowie" Best of Show by Eliza Ollinger", who I suspect doesn't know where her delete button is but seems to have a talent for coming up with a catchy title.
Opinions?



