If your opponent attacks your weak group and has a (related) weak group himself, attacking his weak group may be/is often a better idea than trying to run or live directly with your group. Some possible scenarios:The best defence is a good offence
You answer his attack directly. The strength he gains hounding your weak group helps his weak group.
You attack his weak group in answer. He follows up against your weak group. You follow up against his weak group.
You attack his weak group in answer, he defends his weak group. You use this to help your group.
And very relatedly-
Don't start an attack when you have left a weakness that compromises it.Close fist before striking
