If we are talking about healthy people here then I would have to say the key for me would be mature emotional intimacy based on honesty, empathy and trust. It reminds me of the 5 languages of love which I think misses the boat entirely on what true love is. I feel all of those 5 "languages".. words of affirmation, quality time, gifts, service, and physical touch are all just "acts" of affection. They mean nothing to me without intimacy. But not physical intimacy mind you... emotional. What glues it all together is the bond or connection that an emotionally mature, introspective, empathic person can bring to a relationship. You can tell them anything and know they will not judge you harshly for it and will be there for you. They may not always agree with you or want to face up to problems you have, but you know they will try to help you both get through it. And you will do the same for them. An emotionally immature person will leave you feeling as alone as if you were lost in the middle of the Sahara. You crawl to them like an oasis but you are always left thirsty. And the narcissist is the worst because they are a mirage. They pretend to be the oasis you need but hold poisoned water if any.