The hunter and fisherman 

A hunter returning with his dogs and his product if met a fisherman who also returned from his fishing, both with their baskets full. 

Hunter wished to have the fish, and the owner of the fish meat. They soon agreed exchanged baskets. The two were so pleased your treatment for a long time continued to do so every day. 

Finally a neighbor advised them: 

If continue like this, the time will come for such frequent exchange, ruin the pleasure of it, and everyone will want to keep only what he got. 

Varies and alternates to better enjoy your activities.