It is widely explained that smiling is a lot easier on the facial muscles than frowning. This may not be a good enough reason to be happy, but it's nice to know...
Something that really works - and a better reason to smile - is that displaying a positive, joyful attitude to all we encounter, is infectious. Others catch it, and you will find yourself being happier.
Strangely, many people actually seem to prefer keeping happiness at bay by bemoaning their misfortune at every opportunity.
Surely it is better to reflect on what we DO have rather than what we DON'T have?
There is an enormous amount of negativity around us all the time. It starts when we are children, and unfortunately continues until we die. We need to use every opportunity to offset all the negative energy we've absorbed so far and to fend off future negativity.
Happiness is, in itself, a subject worthy of volumes; but once we are both giving and receiving Love, Peace, Freedom and Harmony, then we will be happy.