What is this unwritten rule of not upvoting your own posts?

I am kind of new to Reddit and I noticed, that most people seem to show resentment for self-upvoting. I tried to find answers online but the best I found contradicting to what I saw. According to Reddit, your own posts are upvoted automatically because the original idea was, that you, as author, must endorse your post or else you would not have created it. While people apparently think that I just like to nibble on my own genitals for pleasure, I have not yet seen or heard anything that convinces me to stop doing it.

I would be happy and thankful if someone could explain this to me!