Among the important qualities you should cultivate is the ability to trust. We will not know what our spouses, friends or partners are thinking, this means we need to trust them. Though not blind trust from the onset but we should learn to at least give people the benefit of the doubt.
The great thing about strong relationships is that you get to understand the person to the point where you can actually mimic their thought process. But before then, be patient as you try to figure each other out and create the blueprint for a good friendship.
Everybody loves surprises so remember to do thoughtful things to show that you had them in your thought and care about them. It does not have to be a gift, just approaching them in any unexpected (positive) way such as a phone call or a letter will do the trick.
Understand that you need to constantly communicate with your friend or partner if you want to grow a good relationship. Good communication extends to the rough periods as well. When you disagree and hurt each other, it is good communication that will help you sort things out and further grow your friendship.
Visualize your ideal relationship
Figure out what you want in a relationship, what qualities are important to you and what qualities you can do without. This will enable you get an idea of how your relationship should be.
Learn to accept the other party
We are not perfect, so trying to seek perfection in others is hypocritical and at worst dangerous. We must learn to accept people for who they are. We can aspire them to be better people but we can only encourage them. Any attempt to manipulate or forcefully bend the other person to your designs will not end well for you.