For that specific example, it's usually because people want someone to stick around, have a drink, and socialize with them.
If someone does not have the time or does not want to drink, all they have to do is say no - to sticking around at all, or to having a drink. Pretty simple. Some people don't drink, or at times don't want to for a variety of reasons like having other things to do that they want to be sober for.
But in general people who are having a drink and feeling good want others to share in that relaxed feeling and invite them to do so vs. be so rigid and serious. It's a "stop and smell the roses," sort of thing.
..buuuut also, some of those people are habitual alcoholics and they just want others to join in their drinking to justify their own alcoholism by having others join them and pretending that their habit is ok/healthy etc as they have others around them joining in.. making them feel okay about being a drunk instead of isolated as an alcoholic drinking solo w/o friends around.
Just totally depends on the person and the context which it might be. Or a combination of. Still, people are free to make their own decision and just say "Nah, I have plans to go workout, do some studying, meet so and so for dinner etc/whatever," and then simply not consume alcohol. People LIKE people joining them for a drink, but no one is forcing them to.
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No

for supporting trump. Because doing so is deplorable.