"One should respect his motherland, his culture and his mother tongue because they are giver of happiness".
Rig Veda
The skirt of the world is woven from the many languages of the world. The skirt in itself is patched like a gypsy's skirt representing various ethnicities that fabricated it in the first place.
Each patch signifies a language , a people, an identity. Your mother tongue gives you your identity.
I'll admit, I initially had no intentions of knowing my mother tongue at all. It had no relevance or importance to me for the first 17 years of my life. Upon leaving my comfort zone and venturing into different grounds, I suddenly realize the importance of knowing your mother tongue.
It becomes a SECRET LANGUAGE. As the minority race, knowing your mother tongue enables secret messages to be passed from a pair of lips to another, undecipherable, sometimes even by their own people. It's awesome, for jokes, complaints or just to annoy another of a different skin tone. Though it may come across as rude sometimes, even racist, I guess there's a degree of wisdom that should be used. Don't speak it if there are other races around in the conversation, though I beg to differ, I fall into temptation many a time myself. ;)
It is the avenue for ANGER. Face it, English just doesn't cut it. Every swear word in English is so watered down and common. When you're mad, you really want others to know you're MAD. That's why I find my Chinese friends converse with me in English but when they're mad, they move over to Cantonese. Similarly, saying 'Get lost' in English is not as passionate as Tamil. Ever watched a Hokkien or Spanish drama? Watch the scenes of anger and bitter hate and you'll get my point. (You do not want to be scolded in Hokkien.)
It is the language of PASSION too. Have someone read you a love poem in Tamil with feelings and all, you'd fall in love. Hearing Bible verses read in Tamil is amazingly wonderful. You might not understand the true meaning of the words but the sounds and syllables of each and every word has expression. A language based on sound is sweet to the ears. What about Bharathiar's patriotic poems! The flare and rhythm within the words still ignites the fire of many until today.
Despite all reasons mentioned, the most important reason one should know their mother tongue is because it makes a part of you, you.
Never forget your roots.