According to the Buda teachings, who have anger or sorrow suffers and therefore it is unhappy. It is completely logical and easy to understand. Therefore It easy to see that “kindness = happiness”; in other words, if you act always with kindness you will always perform happy. So do so.
However, you can ask, have I to perform happy if I am not? My answer is Yes. Why? To answer this first answer we have to think, what is “happiness”? It is either “being happy” or “feeling happy”?
If you think that “feeling happy” is to be happy, then you only will be a few times happy in a random day, or even none. Because feelings are beyond of our control they are out there (good and bad) and only the good feelings “will make you happy”. Indeed, it is what we are programmed to do (due to our natural instincts) and it is what the animals do.
But “being happy” is an mind state. You could be happy always if your mind say so, no matter what is happening. In other words, accept life as it is, not as it would like to be, being happy all the time and you will be happy 
Another reason why we have to behave happy is because we are what we think, eventually we become to be the person that we created in our mind, so create a happy person.
This is what on the book “Way of the peaceful warrior” calls: “unreasonable happiness”; just because.
To recap, remember to act always with kindness (and gratitude) and easily you will live in a state of “BEING HAPPY” (not “FEELING HAPPY”), specially in difficult moments. When we act like this in this difficult moments or ordinary moments (i.e. going to work in the early morning!) our personally grows towards our final goal: permanent happiness.
