1. Everything in your dreams requires your attention to exist.
While this may seem obvious, it's still one of the most basic principles of dreaming. The only things that exist in a dream are things that you are aware of. It's not complete world that you wander around in and explore. As soon as you stop paying attention to something and forget about it, it ceases to exist. If you are in a closed room and can't see the outside, then the outside doesn't exist. When you open to door to go outside, it could lead anywhere, into space or hell or the city of the mole people.
The point is, nothing exists in your dreams until you become aware of it. Much like Schrodinger's Cat.
Schrödinger's cat - Wikipedia, the free encyclopedia
This can be a useful tool I've used effectively in the past to deal enemies while lucid. If you don't pay something any attention, it will cease to exist. Kind of like when your parents told you as a kid to just ignore whoever is bothering you (I hate it when they're right!).
The skill to be learned here is to learn what things you pay attention to. Ask different people who walked down the same street what they saw, and a bird watcher would tell you about the rare wood pecker he saw, a child would tell you about the playground they passed, your wife would tell you about those shoes on sale, and I would tell you about that girl in the short skirt. ;) What are the things that captivate your attention, and how do these things make you feel? How do they affect your life?
Some things you pay attention to are beneficial, and some are detrimental. If you are prone to only noticing the bad in people, then interacting with people is going to be a pain in the ass. If you focus more on the good in people, then it can be a rewarding experience.
You have to learn to selectively ignore the things that don't do you any good, and focus on those things that accomplish your goals. During a dream, the effects of doing so are quite drastic, but practicing this while awake will have echoed results in your dreams, as well as be beneficial to you on several levels in RL.