If a person suddenly starts drinking a lot of alcohol what could be the reasons?

My guy friend has been drinking so much alcohol that he even comes to office drunk and comes late to office because he had been drinking the previous night and even leaves early to drink. He was very dedicated towards his work but now he doesn't care about work as well. He also has stopped hanging out with his old friends and only hangs out with his drinking pal.
If a person suddenly starts drinking a lot of alcohol what could be the reasons?
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