One of the nastiest, stomach knotting feelings is when you lose a fly... IN A FISH! (We all do it occasionally don't we?) The discomfort comes from a pricked conscience. Each of us knows without doubt that we have just done harm, the very last thing any true angler wants.
The obverse of this coin is when you catch a fish and on removing your fly you find there is another, which you remove and joyfully release the fish knowing it is going back better off than it was prior to your success just now. The feeling is one of relief on those occasions when you look closely at the fly to see - Lo and Behold! It is your fly that you lost in that fish on a previous occasion.