How is Lennie miserable in "Of Mice and Men"?

The character of Lennie in the novel Of Mice and
Men, by John Steinbeck is not necessarily meant to appear miserable.
Additionally, there is not enough information in the story that would help us conclude
officially that he was, indeed, miserable. In fact, if you compare Lennie to George,
Crooks, and many other in the novel, he may probably prove to be the least miserable of
them all precisely because he does not know any better. Yet, there are several
characteristics that Lennie possesses that would make a man with average intelligence
feel quite miserable. Then again, Lennie is not a man with average
intelligence.

The problem with Lennie is that he cannot
control himself. He is extremely big and strong for his own good. He is also taller than
his peers. To top it all, he is highly emotional and loses his temper to the point of
hurting, breaking, and even killing by accident. Should a man with a higher level of
intelligence go through this situation, his life would be miserable for sure. It would
be no different than being a type of Dr. Jekyll and Mr. Hyde, or even a type of
Incredible Hulk.

Yet, Lennie only goes as far as knowing
that someone will be mad at him because he did "something" wrong. He does not know what,
or the consequences of what he did, but he knows that he has messed up somehow. However,
it is precisely this innocence what protects Lennie from becoming miserable:
Unfortunately he makes everyone around him, especially George, miserable indeed. For
Lennie is a danger to society, no matter how immature or mentally challenged he may be.
This is the biggest tragedy of his life.