It's not good for health so I do not.
Also it's hard to stop after you've started, - I do not like that. People feel the regular need to get another cigaretes after a short time. They annoy by the smoke the others who do not smoke. Do significant spendings. A slavery with doubtful good use.
It's a drug. Only lesser destructive than restricted already.

Also I do not drink alcohol often or much by the same reasons. I prefer this on celebrations which are 1 per 1-3 monthes, sometimes on weekends. Prefer to keep spirit in ~30-40 g per day. As I do no drink often, I prefer costy kinds alike better wines.

Never used drugs.

There are different not good dependencies which people have besides that.
I like sweets, for example. So I need to force myself to limit them to keep the weight in norma. I remove any meal limitations on Sundays.