Although it is very likely to be true that strong and secure attachment help you lead a better life I would like to emphasize their coexistence more than the potential causual relationship. Anyone who is in a postive relationship is likely to be in a good and happy life becuase of it. No matter how happy we are, without any trustworthy relationship, our happiness cannot be complete - as social beings, we need interactions with others and our satisfaction increases as they grow firmer. Therefore, a direct casuation between strong attchment and happiness may not be but it is true that they exist together most of the time.
Then, the question is whether a healthy attachment causes happiness later in life. As I have established before, happiness is accompanied by strong attachment; to be happy later you also need a good relationship later. And it is obvious that one who were in such relationship knows that it makes one happier and is more likely to dare bonding strongly again. I see here a learning process: that when one forms strong attachment he is rewarded happiness. Therefore, the actual causation between better later life and strong attachement must be learning from experience.
