1) The first reason is that people tend to judge themselves to be better than they are across the board. So more people will thing they are above average than below average no matter what we are talking about, poetry or cooking....
2) The second reason is that you confuse your own emotion with the poem that is the result of it. So everything you felt when writing it, you think is in the poem, including the effort or the excitement of writing it. But the reader just sees the words, with no access to your feelings. If you look at poems you wrote a long time ago, having forgot the emotion of writing it, you will see that those poems from the position of a reader who doesn't know you.
3) The third reason is that your poem could be good, in fact, but that it is good mostly for someone with your own taste. This third reason is actually not a reason why your poem isn't good, but simply a reason why other people might not share your opinion. They might not only dislike your work, but also your favorite poet.
For # 3, you probably shouldn't care whether people of opposite preference will like what you write. You can't do much about #1, either. That's just human nature. For # 2, it's just important not to confuse the emotion behind the poem with the possible emotion for someone else reading it.