1. He is seeking revenge for his father's death: Laertes has learned from his sister Ophelia that their father, Polonius, was killed by Hamlet. Consumed by grief and rage, he demands justice from the King and accuses him of complicity in the murder. He wants the King to take action against Hamlet and avenge Polonius' death.
2. He is manipulated by Claudius: Claudius, fearing Laertes' potential for revenge, has been subtly manipulating him. He feeds Laertes false information about Polonius' death, implying that it was a premeditated act by Hamlet and that the King is powerless to act. This fuels Laertes' anger and makes him more susceptible to the King's plans.
In short, Laertes' forceful entry is driven by his desire for revenge and fueled by Claudius' manipulations. He enters to confront the King, demand justice for his father, and ultimately, become a tool in Claudius' scheme to get rid of Hamlet.