My Friend Just Got Out of Rehab and He Still Seems Sad. What Am I Going to Do?

Bobby just completed the entire drug treatment therapy in a drug rehabilitation center. The counselors and the doctors said that he did just fine. He was cooperative and participative in the activities and therapies. I was happy to hear the news and I was also excited to have my friend back. The thing is, when I first saw him after his exit from rehab, I noticed the sad look in his eyes. He still seems depressed and miserable that I’m scared that he will go back into his old ways and rely on his other best friend: marijuana. What am I supposed to do? I really want to help Bobby overcome his addiction.