Secrets to a happy relationship

I feel that in order to maintain strong and lasting relationships people have to be unselfish and be as concerned about their partner's happiness as their own. They have to be in tune to their partner's needs and feelings and must realize that everyone is constantly changing and growing and everyone, no matter who they are, needs to be loved, appreciated and understood just the way they are.  I feel that many people nowadays are very selfish and only concerned about their changing whims and desires.  I would like your input on this.