Why does Siddhartha experience discontent with life at the start of his journey? 

Hesse depicts Siddhartha as unhappy with life because it
lacks the real sense of meaning that he so desperately seeks.  Siddhartha is shown to be
precocious and one who seeks to understand more than what is there.  This deeper
striving for meaning is what causes him discontent.  It causes him discontent because he
is unable to simply capitulate to what others do and what others have prescribed for
him.  In this, Siddhartha is shown to be different than others, a non- conformist
immersed in a world of conformity and acquiescence to socially dictated notions of the
good.  The severe and intense strain of questioning to find meaning is what drives
Siddhartha to want more and to seek more out of reality and out of what is there.  His
need to want to know more and to want to gain a greater understanding to meaning in his
own life is what represents the discontent that he experiences.  It is this fundamental
discontent that encourages him to leave his home with Govinda and find meaning,
discovering a different path than what is there.  In his yearning to see what can be as
opposed to what is, Siddhartha feels discontent at being immersed in a world of the
latter.  His journey to achieve the former is the result of this
discontent.