1) Communication // Talking about important matters, no matter how tricky, is truly essential in any relationship. If someone does something to hurt you, rather than "sweep it under the rug" so to speak, it's important to bring it up right then and there rather than let any resentment build up! When you can express your feelings to the other person them feeling like you are complaining about them or that you are attacking them, this is a sign that the person is mature enough to handle a mature, adult relationship.
2) Acceptance // Understanding that everyone has their flaws - including you! - is necessary for accepting other people who they are, rather than trying to change them or mold them to fit some ideal person that you want to turn them into. Maybe your best friend will never arrive exactly on time when you're supposed to meet for your weekly coffee dates, or your boyfriend will never wear Lacoste polo shirts unless you buy them for him, but letting these somewhat unfortunate quirks go is an important part of showing your love for someone. The important thing is knowing your non-negotiables, because there are certain things you should never accept (exhibit A: abuse of any kind). But, generally speaking, live and let live, you know!?
What are the key components that make your relationships work?