John Cassian was aware that we often do not know how attached we are to something until we find ourselves deprived of it. The resulting emotional fallout can surprise us. This is precisely the reaction Cassian is trying to provoke. This shows us how much our heart was filled with whatever material thing or things we are now trying to do without.
Cassian can seem to be quite radical about dispossession, but it is worth keeping in mind that he is not talking about destitution. He is happy for us to possess those material goods that are necessary for our life, but if he were alive today, I am sure he would say, along with Pope Francis, that becoming free of “compulsive consumerism” through renunciation is vital as a first step to gaining purity of heart, and to entering into a new relationship with Christ, who was poor (Institutes 4:V).
The one who practices renunciation gives Christ greater access to his heart. Although the person practising renunciation may have fewer material possessions than others, he is not bereft of what is most valuable, for what is most valuable is his relationship to Christ.