why i person falls in addiction? specially drugs!!

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we have various people arround us. i have a friend named mamun. he is same aged to me. i like him very much because he is my chilhood friend. once he went abroad fro further study. and came back after three years. in the mean time i had also complited my studies and could find a job. but he couldnt get one. that made him frustrated. once he came to me and asked me to give him some money.i didnt think anything but give him 500bucks and he went away keeping ht emoney in his pocket. he didnt tell me the reason opf taking the money. after few hours i had a phone call from his house . his elder brother was informing that the police has caught him fro taking drugs. Marijuana. i was shocked. my friend took the money for to buy drugs. i couldnt beleive that. the question arised in my mind that why did he do that???