I would assume that this is because he is her only child and he is growing toward manhood.  Because of this, she has to treat him somewhat cautiously.

On the one hand, she wants to continue to keep an eye on him.  She wants to be sure that he is safe because he is all that she seems to have in the world.  But on the other hand she seems to know that if she is too controlling she is going to stunt his development and/or she is going to drive him to rebel.

So she has to walk this path between being too strict and too lenient.