Why does Hamlet attack Laertes?

Answer 1

"Osric did not "get" Hamlet and Laertes to fight. He stood by as a referee in the duel between the two adversaries. He called out the "hits". They duelled because Laertes believed that Hamlet was responsible for the death of his sister Ophelia and wished to avenge it. This belief had been fostered by Claudius who saw a way of having Hamlet killed as he knew that Claudius had murdered his - Hamlet's - father."

This is almost correct. Indeed Osric was not the cause of Hamlet and Laertes duel, the King was the one who came up with this idea because he found it the easiest way to have Hamlet killed without the suspect of foul play. But Laertes agreed to the duel not only because he believed Hamlet's madness caused Ophelia to also go mad ultimately killing her, he also dueled because of Hamlet's cold blooded murder of his father Polonius...just wanted to add that Polonius had two reasons for wanting to duel Hamlet.

Answer 2

Which time? He does it twice--the first time when Laertes has jumped into Ophelia's grave. Hamlet is angered by this ridiculous (and as he knows, less than fully sincere) show of affection and attacks Laertes. The second time is after he is wounded in the duel. Hamlet, realizing that since Laertes is fighting with a real unbated sword, this is no game but a genuine fight, takes the initiative and attacks Laertes to make him drop the sword.

Answer 3

Laertes is looking to avenge the death of his father and sister (Polonius & Ophelia). Hamlet is considered to have been responsible for both of their deaths, thus Laertes looks to kill Hamlet in return.

Answer 4

Laertes wanted revenge against Hamlet because Hamlet killed his father, Polonius.