(Please note, this is written from a male perspective. It can easily be switched to a female/other perspective by flipping pronouns. Unless that's not your thing.)
At the core, every relationship is about equality. Sharing. Relating. This might seem odd to some, those who don't truly get the concept.
Not just sexual, "physical", or romantic relationships either. Friendship is a relationship.
It's all about sharing. When with a woman (not a girl, that is a whole 'nother monster), a man (note, man, not boy), must do the following.
1. Communicate. Talk to her, damn it. She's not a fucking psychic. This doesn't mean constantly spewing "You're beautiful" or "I love you". She knows. That shit is tired. Actually saying things to her is communication. Example: Your significant other walks in from work. "How was work?" You ask. Not original at all, but a jumping point; this goes somewhere. "Horrible." She replies. "I hate working there." She's setting you up. Now for the single most powerful word, just three letters, in the entire realm of communication. "Why?" You ask. This blows the whole damn thing open. She'll give reasons (if she doesn't, inquire further, unless given signals to drop it) and you should note things, comment on feeling and fact of her reasons, reply with opinions and facts of your own. Don't fall into the "Yes Dear/Uh huh" rut. Both of those will kill the conversation, right then and there.
2. Give a shit. This may seem obvious, but many people don't. You should care, in many ways. You should focus on this person. That doesn't mean drop everything for this person. Live your life. If they feel bad, talk about it. Communicate. There it is again. In everything from casual conversation to sex, you have to think of them, not you. Just remember to have time for yourself.
3. Want their happiness. Another which is easily misconstrued. Don't devote your entire life to making them happy, you have to make time for yourself, but try when you can. A friend might need someone to talk to, and you can make their day a little brighter. Or, your significant other decides to mount you before bed. She's not just there for you to get off, although that should be one of two priorities. She wants to get you and her off. You can give her happiness the same way she does, through orgasm. This should be the man's main priority, and the woman's should be to do the same for him. Equilibrium. Note that this is only one way, and by no means should sex have to be a major part in a relationship at all times. The same rule applies though. Give what you get, keep it balanced.
If one adopts these mentalities
, one will experience more personal relationships, closer friends, and happier love-lives.