how necessary is saying "THANK YOU"?

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are you particular in saying "thank you"? well,me..i am very particular with that word.'coz i lived with that word everyday. saying thanks is a sign of your gratitude. So,how can you let the person know that you are grateful with what he/she's doing? Let me share you something,I have this one friend--a good friend(probably)hehehehe.but anyway,so much for that. this friend is a girl,and i have observed that she is always asking something,mandating. every time she gets what she ordered,she never says "thank you".(that's all) I can't just imagine how she sleeps at night,without saying thank you with all her demands.my gush!