Why does Hamlet change?
When he makes this remark, he is fresh off successfully out-scheming Claudius and saving his own skin with an elaborate ruse. It's true he's also been overthinking things to the point of inaction, but why go so far in the other direction? Now he's disregarding his own heartsickness and Horatio's advice to skip the duel with Laertes.
I actually liked the Polonius-stabbing, Ophelia-destroying, excuse-making Hamlet better than this drip who equates clear signs of danger with mere "augury" and contemplates little birdies.
Oh well, is that our enigmatic Hamlet again? Can't settle on a reasonable medium, has to seesaw between extremes?