This is why doctors should recommend drinking and smoking. At a certain point the risk of premature death is greater from being a shut-in than it is from indulging in a pint and a ciggie with some buddies every once in a while.
People can and do drink and smoke alone. Indeed, lonely people may turn to alcohol for self-medication. And of course those addicted to nicotine have to smoke regardless of whether other people are around. That will just make the problem worse.
Besides, smoking has largely been banned from buildings, so that habit doesn't make it convenient to gather with others, except perhaps huddled outside the door.
But I don't think it makes a diff. My friend circle has had a few guys drop into hermit mode. We have tried all kinds of things to pull them out, but it's like they have decided monastic life or whatever is their thing.
Besides, smoking has largely been banned from buildings, so that habit doesn't make it convenient to gather with others, except perhaps huddled outside the door.