I have a theory about why I think people do this. Deep down they want a partner in crime. The ones who do this really don't feel that drinking is OK (even though they may not admit that to themselves) and feel better if they get others to do it.

That said I have never understood the need to get others to join you in your activities, but I am by nature not a joiner and hate telling or convincing others to do something.

I like to drink, but when someone tries to convince me to, it becomes a personal challenge to do the opposite :-)

I can be contrary.

In my personal experience, drinking around workmates is fraught with traps and try to avoid it.