I, too, disagree with this statement. As lynnebh notes, Lennie's problem is his mental weakness, which prevents him from understanding his own strength and learning how to control it.
In many regards, Lennie's physical strength is an asset. Because Lennie is virtually incapable of answering questions for himself, George, when the two apply for jobs, points out Lennie's physical strength to prospective employers. Because the work of ranch-hands consists of manual labor, Lennie's strength helps him to do his job well.
Never in the novel does Lennie seek to harm another person or animal. He accidentally kills the puppy, he hurts Curley in self-defense and only after George gives him permission to fight back, and he accidentally snaps Curley's wife's neck when he becomes panicked by her screaming.
So while it may seem that Lennie's strength causes harm to others, readers to evaluate the novel closely will understand that Lennie's mental limitations get him into trouble, not his strength.
I would have to disagree. I think his mental weakness is his worst enemy. One can have enormous physical strength such as Lennie does but if one has all one's mental faculties, then one could keep the physical strength under control. In Lennie's case, his mental strength does not equal his physical strength so he does not understand how dangerous his physical strength can be if he does not control it. This is why when he thinks he is playing with little animals (mice) and being kind to them and petting them, he winds up killing them. Same thing with humans. He does not realize that his strength can hurt people, especially women. Also, the reason that he and George have had to run away was because of some other trouble Lennie had gotten into earlier on. George must protect Lennie, not because he needs physical protection, but because he needs protection of another sort. George is his caretaker. He tries to prevent Lennie from making mistakes and hurting people with his physical strength because Lennie does not have enough mental strength to do this himself.