Smiling is good for health. It is the sign of a healthy and happy mind and is an indicator of friendly behaviour. And if you work at an airport, you are pretty much needed to do so. It's just polite.

Source: BBC

Unless you are the security force at Indian airports. Then you must not smile as much as you do or you're giving a perception of lax security, leading to terrorist attacks.

Source: Google Plus

Yup. That's what the Central Industrial Security Force (in charge of India's aviation security) said. 

Source: Jetss

The CISF wants its staff to smile less so that it looks, 'more vigilant than friendly', according to the BBC

Source: Rediff

CISF DG Rajesh Ranjan spoke to the Indian Express on the matter. 

We cannot be over-friendly with the passengers because one of the reasons cited as to why 9/11 happened… was excessive reliance on passenger-friendly features where security personnel went out of the way to ensure that the passenger is facilitated, thereby compromising on security.The staff has been instructed to go from a “Broad Smile System” to “Sufficient Smile System” to focus on ensuring foolproof security. 

Source: Gifer

We are not ones to comment on what makes airport security better but asking the staff to smile less does seem a bit peculiar.