There are a number of things that could be argued to be the most important factor in a healthy relationship, but what is the #1 Most Important Factor? It seems that if you knew the key to this, all your relationships would be happy and healthy, and you'd be able make your relationship last forever, right? Trust is always important, but is it the most important thing? Compromise is necessary, but can a relationship survive if on compromise alone? Of course, love is necessary too, but that should never need to be brought to a relationship, only held on to. So what is the most important factor in a healthy relationship?
Honesty. Open and complete honesty.
If you have chosen one person you want to spend the rest of your life with, you should be able to tell them anything and everything. Some professionals (and non-professionals) will argue that a relationship should have its secrets, but I do not believe this is the case. It may be hard to tell your partner something, and perhaps detailed recollections of your past relationships or sexual encounters should be avoided, but if you have something on your mind, weighing on your heart, you should be able to share with your partner.
You may feel that your partner should feel comfortable telling you everything. And if you find that he or she is not, your first reaction may be to feel offended. Why wouldn't they feel comfortable telling you something? You're the one person they SHOULD feel comfortable with! You're the person they will be spending the rest of their life with. But just because they SHOULD be comfortable, doesn't mean they aren't self conscious. And sometimes there are things that it seems harder to share with your partner. Whether it is because you're afraid to disappoint, you feel that you suddenly will not be the man or woman they thought you were, or you do not want to burden your partner - telling your partner is something you just downright dread. As much as it terrifies you, you - and your partner - will feel better for it. If your partner reacts badly, perhaps he or she is not the right person for you - or is not really ready for a long-term relationship.
I recommend, even if you don't feel that your partner might be holding back, you take the time to let them know it IS OK. Take a few minutes, take your partner's hand, and tell him or her that it is OK to say ANYTHING at all, and you will not judge. And if he or she doesn't want to talk right now, or has nothing to say right now, that all he or she has to do is let you know when it is the right time.