"You never know who can fall in love with your smile". That common saying may contain some truth, because if you are always with an angry expression maybe people wouldn't be near you. For many people, humour is a style of life like clowns or humorists. It is a way of liberating tensions and energy, very necessary.
In the first place, people enjoy a lot watching people that are laughing because it is contagious. It's obvious that if you start to laugh people around will be attracted by your laugh and will start feeling desire of laughing too.
In the second place, In my opinion, I believe that sometimes it is better to take life with less seriousness. For example, when we are in a mishap without solution, instead of starting despairing why not to laugh at the situation ironically? Humour can provide the distancing necessary to recover from any situation, though any more than in a few seconds. If we have stumbled over a stone, instead of beginning to exploding of anger or to saying swearwords, we will laugh at our own ridiculous.
Lastly, it has always been said that by laughing we use fewer muscles of the face than putting an expression of anger. Then, shall we take the life with a bit more of happiness? I believe in humour and in laughs of the people because it is something contagious that, sometimes, just by seeing someone laughing of happiness transmits a similar feeling instantly.