What is the relationship between Hamlet and Laertes?

The relationship between Hamlet and Laertes, at least at the beginning of the play, is virtually nonexistent. We never really see the two interact, and when Laertes speaks to his sister, Ophelia, about her relationship with Hamlet, Laertes seems really to only talk about him as the future monarch of Denmark and not as a young man. Laertes says:

His greatness weighed, his will is not his own.For he himself is subject to his birth. He may not, as unvalued persons do,Carve for himself, for on his choice dependsThe safety and the health of this whole state. (1.3.17-21)

If Laertes really knew Hamlet well, he might be able to speak about him in more specific and personal ways; however, it is evident that he really only knows Hamlet as the prince of Denmark and not as a friend.
By the end of the play, however, the relationship between Hamlet and Laertes is quite antagonistic. Hamlet insists that he loved Ophelia more than "forty thousand brothers" could do, and Laertes is enraged by the fact that Hamlet murdered his father, Polonius. When they engage in swordplay, Laertes tips his foil with poison so that if he even scratches the prince, Hamlet will die. Eventually, the two reconcile just prior to their deaths.

Hamlet has been away studying in Wittenberg, Germany, and Laertes has been studying in France before the play begins. Both men meet back in Denmark for a short time, presumably because the death of the former King Hamlet and the marriage and crowning of Claudius and Gertrude bring them both to court. Since they have been living apart from each other, they don't know each other all that well. However, their families are clearly close. Polonius is a chief adviser to Claudius. He is perhaps his most trusted courtier. Further, Hamlet has entered into a flirtation with Laertes's sister Ophelia and appears to be in love with her.
Laertes shows he distrusts Hamlet and his intentions when he warns Ophelia to beware of him. He clearly worries that Hamlet's designs on his sister are not honorable.
Although threads connect the two young men, they are hardly bosom buddies. Laertes is out for blood when he has returned to France and hears that Hamlet has killed his father.

We first meet Laertes in Act I, Scene 3 of Hamlet. Laertes is already on his way to attend the university in France, and he does not appear again until Act IV, Scene 5, after his father has been slain and his sister Ophelia has gone mad. Why does Shakespeare introduce this important character early in the play and then keep him offstage until it is nearing the end? Evidently, Shakespeare intended Hamlet and Laertes to become enemies, as they do when they encounter each other at Ophelia's grave. Shakespeare wanted to avoid showing that the two men enjoyed a cordial relationship, or any kind of relationship, before Laertes went away to France. Such a previous relationship would only complicate matters for the playwright. It would be easier to handle the emotions of both Hamlet and Laertes if they did not have a previous amicable relationship. That explains why Shakespeare sends Laertes off to the university in France and keeps him there on ice, so to speak, until he is needed to play his essential part in the play towards the end. It also could explain the seemingly gratuitous Scene 1 of Act II in which Polonius is sending Laertes money and instructing the messenger Reynaldo about how to spy on his son. Shakespeare's intention might be to establish that Laertes is now in France and to keep the audience aware of Laertes's existence and possible importance to the plot.