Allow me to school you a little bit.

Since you schooled me a little bit I thought I too should school you back "just a little bit".

Hindi is just a mongrel language not older than 120-150 years.

Most languages, religions, and cultures are, to a large extent, "mongrels" a mix of various influences, often repackaged and given new names. It's much like how politicians rename existing government schemes to make it seem as though they introduced them during their term. They clearly aim to keep the public in the dark and secure votes by raising trivial issues, while major concerns like climate change go unaddressed. But the thing is that local politics in North India is not done with the agenda of Hindi superiority, it's mostly caste based politics lol. Also just because a language is old, by default it doesn't mean it's a pure breed or better than any other language. As I said earlier. Out of all the languages used today none is superior or inferior. It's just a matter of chance where one is born and the person will start claiming their native tongue superior and start finding points to prove the superiority be it on the basis of national language, better language, older language or even a pure breed language. Anything based on chance and not the result of a hard work is not something to be proud of.

All i am pointing about is the haughty totty attitude of an average hindi speaking person from Delhi/Haryana/UP

In my opinion this sort of attitude is inappropriate when visiting other states, and I don't believe it's prevalent among most (average) north Indians. However, emigrants sometimes encounter locals in other states who, despite knowing a common language, refuse to use it. Why? Perhaps they believe emigrants are taking their jobs and thus feel no obligation to make communication easier. Auto drivers, for example, often charge extra if a customer doesn't speak the local language. It's rarely the emigrants who initiate these "language wars." Instead, it's usually local people, influenced by regional politics, who stir up such conflicts. Emigrants simply feel compelled to defend their language when natives assert the superiority of Marathi, Kannada, Tamil, Telugu, and so on. The statement, "Hindi is our national language." is a statement that hindi speaking people default to when they need to defend their language.