The question here at that point: What is it that is making our mind rigid? What is it that is really making our mind rigid?
The real cause that is making our mind so rigid, consistently, all the time, in big number, is attachment to transitory phenomena. In this case, it is basically attachment to this life. It is the attachment to this life that is making our mind rigid. There is no flexibility. You are attached to this life so options are very little. There is not so much option. So this is why Shantideva in all his eight chapter of Bodhicaryavatara, from ten directions he attacks attachment to this life. Not only this life but in his text it goes on in many, with attachment to a lot of things.