Justin Bieber will not get drunk because he does not want to be out of control. It seems that his manager Scooter Braun holds the Boyfriend singer to a high standard, which may help too.
Braun said, "He knows that I hold him to a high standard... He doesn't want to blow it." So while Justin Bieber is a completely normal 18-year-old guy who has tried an occasional beer, he does not want to ruin his career, his relationship with Selena Gomez, and his life by doing something stupid and getting out of control. This is good for him because he does not want to lose everything he's worked so hard to gain these past several years.
It seems like the teen heart-throb has surrounded himself with a great group of people who are going to keep him grounded. He has his mother, Scooter Braun, and Selena Gomez to name just a few, and they all seem to genuinely care about him and his future. Nobody wants to see this lovable guy go down in flames, and it seems he has reached adulthood and kept it together, so hopefully that means he is good for the future.