Why did Frodo feel he had to leave Middle Earth at the of ROTK?

he had grown beyond it in mind and stature, and yet he received no praise from the Shire-folk for all his great deeds.

He had grown indeed. Grown so much that he requested that they not be told of those deeds because he did not desire praise. He needed peace, peace from the burning ring in his mind, always there when he shut his eyes, even tho' it had been destroyed.