Buddhism is neither pessimistic nor optimistic. 

If anything at all, it is realistic, for it takes a realistic view of life and the world.

It looks at things objectively (yathābhūtam). 

It does not falsely lull you into living in a fool's paradise,

 nor does it frighten and agonize you with all kinds of imaginary fears and sins.

 It tells you exactly and objectively what you are and what the world around you is,

 and shows you the way to perfect freedom, peace, tranquility and happiness.