>>4569
It is not really sinusoidal function it is mostly random.
we are happy when things are getting better(or expect then to get better in future) and sad when things are getting worse
the faster you climb more happy you are but also more risk you face to faill.

Theoretically you can have only happiness and no sadness if you manage to go up during your all life and then die when you are on the top of your achievements.