WHY?

Sometimes one feels very tired, despite neither being hungry - nor having done some arduous schedule. Sometimes one feels very energetic, after having completed a very hectic schedule, maybe more than 10 hours work over and above daily exercise schedule.
Why?
Sometimes you feel low, despite everything being in place, no worry, nothing to fear...while sometimes you feel happy and carefree, very bold and ready to take on... despite all the situations turning against you - step by step, turn by turn...
Why?
Sometimes one keeps quiet despite knowing all the answers, and sometimes the same one keeps on speaking rapidfire - be it questions only... Why?
And why after all, someone has to ask WHY?
Do other animals also have such inquisitive or curious nature?