The battle is constant - to try to remain independent from emotions that occur within us involuntarily. So many times, people act and react without thought. The discipline of assent teaches us to assert our own will and conclusions over our emotions and reactions. From pleasure to pain, from tranquility to surprise; we should discipline our directing mind to not be swayed by these raw emotions that stir within us.
Regarding this specific passage, Hadot says,
The guiding principle draws a border, as it were, between sensitive emotions and its freedom of judgment, by refusing to consent or give its assent to judgments which would attribute a positive or negative value to the pleasures or pains that occur within the body. This border does not prevent the guiding principle from perceiving everything that goes on within the body, and thereby it ensures the unity of consciousness of the entire living being, just as, within the cosmic living being, everything goes back to the single consciousness of the guiding principle of the universe (IV, 40). From this new perspective, Marcus continues, we cannot prevent sensations from penetrating within the guiding principle, since they are natural phenomena; nevertheless, the guiding principle must not add its own value-judgments concerning them.
(see Citadel p. 116)