Lennie gets into trouble because he is childlike and doesn’t really have any impulse control.
Lennie is a developmentally delayed man who happens to be incredibly strong. He has the mental age of a child. As a result, he often gets into trouble. This is why he travels around with George, who looks out for him, takes care of him, and laments how much trouble he is.
"God, you're a lot of trouble," said George. "I could get along so easy and so nice if I didn't have you on my tail. I could live so easy and maybe have a girl." (Ch. 1)
Lennie’s main problems are that he doesn’t know his own strength and that he likes to touch soft things. This is why they got run out of Weed, a small town in Northern California where Lennie wanted to “feel” a lady’s dress. She thought he wanted something else. She was scared. He was scared because she was scared, and held on to her dress. The next thing you know, George and Lennie were on the run.
This incident foreshadows the trouble Lennie gets into later with Curley and his wife. In the case of Curley, Lennie was just minding his own business, smiling about the ranch he and George would have one day. The other reason Lennie always got into trouble was that he was not aware of the world around him and could not pick up social cues. He had no idea that smiling would be seen as a threat by Curley. Curley picked a fight, and George allowed Lennie to fight back. Lennie got scared. He did not let go of Curley’s hand, smashing it badly.
Smashing Curley’s hand was nothing compared to what Lennie did to his wife. He was minding his own business in the barn, stroking his puppy (who did not survive the encounter) when she came in. She was lonely and wanted someone to talk to. She didn’t realize that he was dangerous because she found his childlike nature disarming. He never meant to be dangerous. However, she offered him her hair to stroke. When he did not let go, she panicked.
"You let go!"
Lennie was in a panic. His face was contorted. She screamed then, and Lennie's other hand closed over her mouth and nose. "Please don't," he begged. "Oh! Please don't do that. George'll be mad." (Ch. 5)
Lennie did not mean to kill her. He got scared, just like with the lady’s dress and with Curley’s hand. Lennie knew enough to go hide where George told him to. After he realized that Curley and the other men would come to kill Lennie, George shot Lennie to spare him from drawn-out suffering. He was protecting him, because he was his responsibility.