Why happiness why sadness the 2 real facets of life make the human being countless to there self?

Though these words sound pretty baffling/mysterious but my dear friends sometimes these are the only thing which makes every part of life wet and dry, faces of both sides of coin. Leaving of somebody and arrival of somebody in this world makes a human being responsible for reasons and yourself the main accused of that reason for anticipation answer to those question which are left by somebody and thought after the arrival of somebody.