Which factors contribute to Hamlet's decision in ACT IV to resolve to seek revenge?

Hamlet finally resolves to get revenge on Claudius in Act 4 when he sees Fortinbras' troops. He finds out that they are fighting for a tiny plot of land. When he thinks about the fact that 20,000 soldiers are willing to give up their lives for something that barely even matters, he is disgusted with his own inability to act and says from now on he will only think of getting revenge.