When you feel low, everyone appears to be stranger from some other world, you feel a sort of vacuum in your life, you feel worthlessness, you feel you could do nothing in life till now, nothing is left in your life, you feel descending in a well or you feel like running away from all out there, you feel the voice of others talking to you irritating. That is the moment you are all alone with yourself.
That is the time you need to think upon a way to create your own happiness, to live meaningful and self driven life rather than leading a pitiable life of depending on others or the circumstances, for happiness.
Happiness is not a momentary joy of possessing more and more, buying more and more, accumulating the material things, but a state of inner, eternal joy, which doesn't depend on any outside factor beyond the control of a person but can be generated by a person himself at any moment.
The instant effort should be made to stretch your lips wide even if you need to do it forcefully, so as to give a smile to someone. Then to feel grateful for what you have been given by God, that you are not capable of purchasing by worldly price like eyesight, perfect parts of body, hearing capacity, sense of smell, sense of taste.....
Just imagine absence of any of these, to appreciate these gifts one by one. Just close your eyes to imagine that, then think about not being able to judge the taste of your favorite dishes, then go further to imagine you are not able to sense the fragrance of flowers, flavor of your tasty food, you are not able to differentiate the taste of different fruits, vegetables and all other eatables you love to eat and drink. Just think about the people who are not able to enjoy even that which you are enjoying.
Be grateful for all that and appreciate what you have .Now, this is enough to make that intentional smile meaningful and to be happy, to start the next moment with full energy.