You could say something along the lines of:
"Hey, Friend, what's so good about weed? Why are you always smoking it?"
If he's smoking it for medical purposes, then just drop it, but if it's illegal and he's doing it to "look cool", then I would further ask questions about it.
Answer:
When someone has high self esteem, they're very happy than when they have low self esteem. When that person has low self esteem they feel sad and left out. But when they have high self esteem they're in high spirits...
