Answer:
To have a happier and fuller life means that you achieve all your goals, having good relations with other, being satisfied with yourself. So, all this requires consciousness, which we can achieve it studying psychology.
So, psychology brings us the tools to know ourselves, and this is a huge step, because a lot of crisis life period is because of this. Also, studying this science you are able to know the ways to fulfil your goals, and to realised when your are not happy, and to identify those things that affects you in a negative way.