What causes the bitterness of Hamlet?

In addition to all of the incidents listed above, there is an intrinsic melancholia in Hamlet as he contemplates suicide in Act I Sc. ii:  "O, that this too too salied flesh would melt...into a dew..."

Further evidence of melancholia is in his meditations upon man in general: "What a piece of work is man...yet to me, what is this quintessence of dust?..."  Later, Hamlet remarks, "There is a doomsday near....A dream itself is a shadow....I have lost all my mirth, forgone exercises...." (Act II, Scene II). 

And, in his bitterness there may also be some misogyny in his attitudes toward Ophelia, and especially his mother:  "Fraility thy name is woman (Act I, Sc. ii), and "O most pernicious woman/O villain, villain, similing damned villain.... "(Act I, Scene v) 

See the second source listed below.