It is wisdom that purifies the mind. It is wisdom that helps us in our life to lessen the causes of suffering and sow the causes for peace and happiness. Sometimes when people get attached to calm, they don't look at the true causes of their suffering, so in their normal life then all these problems come up to bother them again. Then they don't really advance very much in their practice. They depend on conditions, quiet, pleasant people, pleasant experiences and unfortunately when they are able to get very calm they develop also a sense of conceit.
When conceit arises, the heart closes and then the Compassion/Lovingkindness practice suffers. So it is very dangerous, really, if we get too attached to calm, because of this possibility of conceit arising and the closing of the heart, the closing of the wisdom mind, the non-willingness to investigate into Dukkha. That is the biggest danger, the non-willingness to investigate into Dukkha, because unless we are willing to investigate into Dukkha we won't see the true cause of it and we won't find the ending of it.