Lenny is a fairly static character. His personality stays the same, even though his memory seemed to improve during the start. He is basically a large child. He hasn’t been taught to defend himself, and he gets very scared with even the slightest startling thing. When he gets hurt, George tries to help him stand up for himself, but just makes Lennie more scared. Lennie has always done this, as the reason they were run out of Weed was because Lennie touched somebody’s dress, because he liked it, and they screamed.
This scared him, and he locked his muscles, maintaining his grip. They ran off to the police when he finally let go, and said that they had been raped. He may have scared her, but it was not on purpose, and he did not touch her, only the dress. Even when he kills curley’s wife, he doesn’t mean it, he just wants her to be quiet, and tries asking telling, begging, then he shakes her. He doesn’t realize his own strength. This shows how childish he is, if not very educated or bright.
This is why he is sympathetic, he is childish, not very educated, and he gets scared very easily. Hopefully this convinces you as well.