Hamlet appreciates other people's willingness to fight in a war, and is ashamed that that spirit does not lie in himself.
In Act 4, Scene 4 Hamlet learns that Fortinbras' army is going to fight for a piece of land that is basically worth nothing. He sees that the Polish are defending it and it sparks a lot of thought within him. He is amazed that 20,000 soldiers will willingly give their lives for a piece of land that means nothing to them, but he is unable to take action against his father's murderer. Here he states for the first time his admiration and appreciation of Fortinbras for leading the armies:
"Witness this army of such mass and charge
Led by a delicate and tender prince," --the delicate and tender prince is Fortinbras
Additionally at the end, when Fortinbras arrives at Hamlet's castle, he is sure to make his support of Fortinbras taking over Denmark clear because he has such admiration for him: "But I do prophesy the election lights
On Fortinbras: he has my dying voice"