For Ophelia, her father's death is the last straw that sends her over the edge into madness. She has been plagued by the seeming madness of Hamlet. She believed that he loved her and then was convinced by her father that she ought to break things off given that he was thought to be simply using her and would move on when he found a more appropriate match. She is torn by this and then by Hamlet's reaction. Then Hamlet kills her father accidentally and it is too much for her and she lapses into insanity. Eventually she finds a way to throw herself to her death in the stream.
Laertes, on the other hand, vows revenge upon the murderer and once he is told by Claudius that the man who did the deed was in fact Hamlet, he agrees to participate in Claudius' plot to kill Hamlet. He sees Ophelia's grief as "too womanly" and responds in what he feels is a more appropriate masculine manner to seek revenge.