I Hope I Never Forget This

Listen to the mustn'ts, child,
Listen to the don'ts
Listen to the shouldn'ts
The impossibles, the won'ts
Listen to the never have's,
Then listen close to me--
Anything can happen, child,
Anything can be.
-Shel Silverstein

I also really like this one

The Little Boy and the Old Man by Shel Silverstein

Said the little boy, "Sometimes I drop my spoon."
Said the old man, "I do that too."
The little boy whispered, "I wet my pants."
"I do that too," laughed the little old man.
Said the little boy, "I often cry."
The old man nodded, "So do I."
"But worst of all," said the boy, "it seems
Grown-ups don't pay attention to me."
And he felt the warmth of a wrinkled old hand.
"I know what you mean," said the little old man.