Hamlet is a play of love and love lost. The first lost love is Hamlet's father. When Hamlet loses his father, he turns to two people to replace that love: his mother, Gertrude, and Ophelia; both of whom betray his love.
Gertrude betrays that love by marrying her brother-in-law, Claudius, and Ophelia betrays his love by refusing him on her father's advice. This love betrayed leads to anger as Hamlet scorns both of them.
Gertrude too, experiences love lost when she loses her husband, so she turns to Claudius to replace it (just like Hamlet turned to her and Ophelia to replace his). Similarly, her second love betrays her, but by the time she discovers this, she is already dying (after Claudius accidentally poisoned her).
Then, Ophelia experiences love lost after both the loss of her father and her forced refusal of Hamlet. She has no one else to turn to for a second love (remember, Laertes is still away at this point), so she simply goes insane and falls in the river.
Now, Laertes experiences love lost from the deaths of both, his father and his sister. Likewise, he has no one to turn to, so he experiences anger and feelings of betrayal, and decides to lash out at Claudius who he feels was responsible for the two deaths. You could almost call Claudius his second love, since the King wooed Laertes into joining his plot against Hamlet. Laertes then feels betrayed by this so-called second love when he realizes his mistake in the dual, and as a result he tells Hamlet everything he knows.
Two, smaller love lost expamples occur with Fortinbras and the Player Queen.
Fortinbras, like Hamlet has lost a father, but instead of turning to a second love, he decides to vent his anger and sorrow by attacking Denmark and reclaiming the land his father lost to Hamlet's father (ironically, Fortinbras ends up with the whole of Denmark when his army visits Elsinore in V.II).
The Player Queen, as you probably know, is supposed to be a mirror of Gertrude.
She loses her husband, and turns to her brother-in-law for second love. The dumb- show in Act III, Scene II is a perfectly condensed version of this whole love cycle.
"Enter a King and Queen, very lovingly; the Queen embracing him, and he her...The Queen returns, finds the King dead, and makes passionate action...The Poisoner wooes the Queen with gifts; she seems loath and unwilling awhile, but in the end accepts his love."