Questions are only rejected if they are very offensive or contain explicit content. Placing questions under wrong categories are also a bad move. So if it's not offensive towards anyone, contains spam or have explicit content then there are no grounds to delete a question on. I'm sure it's not nearly as many as you may think. But in general I think most people like questions where they are asked something or asked for their opinion. People don't like answering long questions with loads of unnecessary information and I think many people also just get tired of answering the same old questions over and over again. So keep things fresh and unique and place it under the right category! :)
Some questions are really hard to answer, especially the deep personal ones. Ones that you can't really put yourself in the shoes of the other person to answer. These kind of questions I avoid sometimes simply because I don't always have too much to contribute.
almost 1/3 of my Questions are removed and very few of them are about pee...which shouldn't be removed to begin with..but we have some prudish people on here that let their mothers influence too many of their thought processes.o.O
There's over 600 successful questions every day. A tiny tiny fraction are rejected.