Why did Percy send Medusa's head?

From the text:

Grover gazed down from his tree branch. "Look, Percy, I'm not as smart as Annabeth. I'm not as brave as you. But I'm pretty good at reading emotions. You're glad your dad is alive. You feel good that he's claimed you, and part of you wants to make him proud. That's why you mailed Medusa's head to Olympus. You wanted him to notice what you'd done."