I just have to say multiple reasons...
EK forgetting what made them popular!
Crew forgetting what they liked about the job in the first place..
Dubai forgetting why people choose it over so many other cities..
And in general people always thinking the grass is greener on the other side..
(and this applies to both company and employee!!)
And a final world Dubai isnt always what is shown or percieved to be... and can be a rude awakening to some!