On top of that he also didn't know how truly nice being smart would be and it was harder to miss it after he had already experienced it.

Charlie seemed to be much happier though because he didn't know how much pain was going to happen to him when he realized his friends were mean to him, let alone how much it hurt for him to lose all that he had gained, after he had finally had a taste of intelligence.

Ignorance is bliss and he was truly happier without knowing how nice being smart could be and how much he was missing out on. Also, he was happy not knowing how mean his "friends" were being to him and all the pain that occured with him realizing that was too much.