Four PS in four years

In the present NDA govt some ministers are declared frequent changer of their Private Secretaries(PS). Mahesh Sharma, MoS, Culture is said to be one of them. In the last four years he got 4 Private Secretaries, V Purushottam, IAS, was the first PS to Sharma, MoS and served about 16 moths, Ravinder Kumar, IAS , also served approximate 16 months as PS. Navneet Soni, IRS officer was his third PS and appointed as OSD. He could complete 8 months. His fourth PS was Saurabh Jain, IAS, and he also served for about 8 months. What is the reason behind frequent changes? Only govt or the minister can throw some light.