The God of cricket, Sachin Tendulkar used to kiss his helmet after scoring a century. He used to do that because the helmet had the Indian flag on it and everything what the Master Blaster did on the field was to make his country proud.
This tradition first started by Sachin Tenndulkar and has been followed by many others over the years. This includes names like Virat Kohli, Shikhar Dhawan, Yuvraj Singh and Rohit Sharma, among others.
Kohli is continuing Tendulkar's legacy and whenever he scores a hundred he does the same. However, Indian team former captain, Mahendra Singh Dhoni doesn't wear a helmet with the Indian national flag on it.
Actually, a wicket-keeper every now and then has to keep the helmet someplace back at a short distance from him on the ground when he is not wearing it and a National flag is a symbol of great honour and respect, so it can't be placed on ground as the law states. That is a true conduct, you can Google it.
You don't put the helmet with Flag on the ground or don't put the Flag on the helmet at all. Dhoni chose the latter. He is also an army officer, so he must abide by this even more religiously. Cricketers are great examples to be followed by a nation. So he is just setting a good example for everyone to respect the national flag.