The truth is we sometimes are happy, sometimes we're sad, its inevitable!
Forcing ourselves to be happy when we're sad is like eating a poison.
If we feel like we're happy all the time, we're actually doing the same thing over and over.
If we encounter some problems and feel sad, we stop for a while and think why these things are happening.
Thus, through these obstacles and negatives, our minds will find some new things or solutions to solve them.
We learn new things and these adds up towards our goal for the common good.
And we grow more and more. Never under estimate the negative thoughts, negatives feelings, negative situations and all the negatives in life.
Remember, in electricity, the voltage works through positive and negative terminals.
Thinking positive and forcing ourselves to be positive would lead us to ignore other people's sufferings because we're thinking of ourselves all the time that we're always okay and everything is okay regardless of the fact that many are strugglling and mourning and needs our attention.