Yes I believe that a HEALTHY relationship can indeed add a good amunt of happiness in your life, and it's not even up to discussion. Of course, the relationship is going to be one of your sources of happiness, like your job, your friends and your lifestyle were supposed to be. It's important to add tho that you won't find completeness on someone else - a relationship must be two complete people sharing their completeness. That's the idea of the thing, but romance movies and the lack of a strong masculine image for boys ever since the 70s created a generation of "nice guys", and I include myself in this since I'm a single mother son.
So what is necessary to get in a good relationship? First you have to improve yourself, get out of this beta nice guy box that most guys were directly or indirectly taught to get in since they were kids. Second, recognize your worth - you're a man, you make your own way. Being with someone is great, but what leads your life is your direction and your purpose. The woman that is with you can either be by your side, holding hands and letting you guide the relationship (because yes, men and women are equal in the relationship, but men must have an authority, it's completely up to the man do guide the relationship to what he believes it's the best) or separate your ways.
So if you can get in the point that I described, get yourself a good woman that will support you and you will support her, that's awesome, you surely will be happy. If you're incomplete and get yourself a girl, you can be happy for sometime, more than the average guy, but then you'll start feeling miserable as she becomes your main source of happiness. Needless to say that this relationship won't last long, but the main idea is that these relationships, if you see it with the right mindset, will help you grow. That's what happened with me, personally, and I'm glad it happened this way.
I think I read that quote in a book that says "In a relationship, you either grow together or grow apart". I think it says enough of what a good relationship is supposed to be like.