I would just like to say that I am no Scholar, but the very nexct thing Hamlet does, after pledging to avenge his father death is go to Ophelia and as Ophelia puts it:

Lord Hamlet, with his doublet all unbraced;
No hat upon his head; his stockings foul'd,
Ungarter'd, and down-gyved to his ancle;
Pale as his shirt; his knees knocking each other;
And with a look so piteous in purport
As if he had been loosed out of hell
To speak of horrors, -- he comes before me.

He was very much in his right mind at the end of the previous scene pledging friendship to Horatio and Marcellus. I think, after he sees his fathers ghost, he is consumed with the idea of revenge for his father, and "dresses himself up" to look mad and intentionally appears before her as such. I put it to you that perhaps he wants everyone to think that he is merely lovesick as a cloak for his true intention: finding proof of his fathers murder and revenging it.