Because the procedures to be applied are the same that the ones people use when they think.
You can see that at most solved exercises in this manual. Express the sequents by words, tell them to someone, and after some time he/she will be saying ``of course it's like that, since ...''. You will see that anyone is able to explain how to use some of the 9 derivation rules, even without knowing their name or existence.
For this reason, to discover the solution to a natural deduction problem, forget about introduction and elimination rules, and think normally, changing the letters to simple actions if necessary. It can help to think of concepts like it rains, it doesn't rain, it's sunny, I don't get wet, ... since they are short words and, moreover, everyone has a clear understanding of what does happen when it rains, and can rapidly relate not getting wet with being sunny and not raining, or even more complex formulas.
Daniel Clemente Laboreo 2005-05-17