Adapted from a recent online discussion.
Question: The type of thing where an unwelcome friend invites herself along on a vacation happens to my husband and me occasionally. While he takes it in stride, it drives me nuts! I would never think to say to someone, "Your Saturday plans sound great, I'll come along!"
At the very least, if you're going to do this, ask, don't just assume. How do people even think this is OK? Or are they just not thinking? - Inviting Myself Along
Answer: I suspect 2. Not thinking. Or, 2a. Feeling confident in the friendship, and not thinking about the possibility that people can like you bunches and still have good reasons not to have you along on their trip.
Also, for everyone who feels too comfortable saying, "I'll come along!," there's someone else who doesn't feel comfortable enough saying, "Ooh, I'm sorry, this isn't the best time for that - but I'd love to set up something else with you for next weekend."
Granted, it feels a lot harder to be the one who says no, but the self-inviter is the one taking the bigger emotional risk. The answer you come up with needs to take that into account - decency demands it - which includes saying "no" when you genuinely don't want that person along (and therefore won't show the kind of enthusiasm a friend has a right to expect).
Re: Inviting yourself:
On the other hand, sometimes inviting yourself is part of the culture. Think college. It took great strength of will to overcome my introverted aversion to saying, "Mind if I tag along?" But if I hadn't, I never would have done anything fun, ever. Though I think I always asked vs. just announcing.
Invitations didn't happen, and people assumed that if you wanted to partake, you'd speak up. It doesn't have to be a bad thing, although it sounds like the original letter-writer's friend didn't have a good sense of where the line is. I still do it occasionally, when it's with someone I'm close to - someone who, I hope, would know that I'd much rather be told "no" than be an imposition. - Anonymous
Answer: Thanks for bringing this up, because I started to include it in my original answer but quit when I realized the mechanics of "tagging along" - specifically, as you say, "where the line is" - were complicated enough to warrant a separate answer. Yes, part of it is the culture - but also part is in the nature of the adventure you're joining. If it's a run for a cup of coffee, OK - but a several-day trip is well over that line.
There's also one's standing with the crowd to consider.
If it's a group of people with whom you're often included, then that's less of an imposition risk than if they include you only when it's your idea.
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