Simple. The Cardassians need the Federation's help and Picard is reluctant to give it because of past experience. Through finding shared pain they heal and create a foundation for a much better relationship between the two powers.Cardassian torture likely happened to thousands of cadets, besides the basic features of truama, what plot potentiol does Cardassian torture have?
I love focusing on emotion so I'll welcome any sort of exploration because for too long it's been brushed under the rug and treated with a *shrug*.