Why was Lennie frustrated in Of Mice and Men?

Lennie is frustrated because George doesn’t allow him to have pets.  He has trouble remembering things.  He is also frustrated when Curley picks on him, because he doesn’t understand what is happening. 

Before George and Lennie get to the ranch, they stop to rest and Lennie finds a mouse.  George knows he has it and makes him throw the dead mouse away, but this frustrates Lennie.  He wants to pet the mouse. 

Lennie's closed hand slowly obeyed. George took the mouse and threw it across the pool to the other side, among the brush. "What you want of a dead mouse, anyways?"

"I could pet it with my thumb while we walked along," said Lennie. (Ch. 1) 

Later, Lennie gets a puppy and the same thing happens.  He kills it by accident and Curley’s wife finds him stroking a dead puppy.  She doesn’t realize that his wanting to pet things gets him into trouble.  They were run out of Weed for him touching a lady’s dress.  He accidentally kills Curley’s wife too, breaking her neck because he wants to stroke her hair. 

Lennie also gets frustrated when he can’t remember things.  George looks out for him.  He knows where they are going and speaks for Lennie when they get there. 

"…You remember about us goin' in to Murray and Ready's, and they give us work cards and bus tickets?"

"Oh, sure, George. I remember that now." His hands went quickly into his side coat pockets. He said gently, "George... I ain't got mine. I musta lost it." He looked down at the ground in despair. (Ch. 1) 

George tells him he didn’t lose his work card, he just never had it.  He wouldn’t let him keep his own work card, because he would lose it.  Lennie doesn’t mind this though, because George is looking after him. 

Lennie gets into trouble again with Curley.  George tells him not to mess with Curley, because Curley wants trouble.  Lennie tries to listen, but Curley sees him smiling and picks a fight.  Frustrated, Lennie fights back when George gives him permission. 

George shouted over and over. "Leggo his hand, Lennie. Leggo. Slim, come help me while the guy got any hand left."

Suddenly Lennie let go his hold. He crouched cowering against the wall. "You tol' me to, George," he said miserably. (Ch. 3) 

Lennie doesn’t understand the world around him.  He is a mild-mannered, kind man.  However, he is very strong.  He was able to injure Curley badly just by hanging on to him.  He was frightened and frustrated by this incident too.