People need variety in their roles, just like in their life.
Napoleon a man known to have affairs was at a sermon at Notre Dame on marital fidelity. Afterwards he invited the Cardinal to the country for the weekend. He served him with pheasant for breakfast, lunch, tea and supper. The cardinal said "Pheasant is a truly magnificent dish - but always pheasant?" Napoleon looked and said "Always Josephine?"
Now I am not saying affairs are good ! I am saying that no matter how great the job is when you appoint someone to the role, unless you bring variety to it, they will get bored, stale and usually leave.
How do you add value to people's roles? How do you extend their learning's or do you just let them get sick of pheasant!