Why does Hamlet tell Horatio everything at the start of the scene?

Hamlet wants Horatio to understand his actions and explains what happened on his journey to England. He says that he strongly suspected Rosencrantz and Guildenstern of foul play, and so decided to apprehend their letter to England. In the letter he found an order for his death. Hamlet then devised a substitute letter asking for the deaths of Rosencrantz and Guildenstern. He happened to have a signet ring in the shape of the seal of Denmark, and so sealed the letter. Hamlet then replaced the letter while Rosencrantz and Guildenstern were asleep. At this point, pirates attacked the vessel, as related previously.