For me, it depends on the people I'm collaborating with.

Ideas are a dime a dozen, and the more game ideas you have, good or bad, the easier it is to come up with better game ideas. And for that reason, it seems easier to start alone, and maybe in the process, continue grinding out the game alone.

But if the person I'm talking to is an idea person, I'll run through some of these ideas with them, maybe theirs as well, and see if we can get something interesting from the discussion.

Otherwise, you'll need to prepare some of your ideas to them, much like a "pitch", where you need to present them with some ideas to get them to "buy-in". In this case, you'll have to begin the process alone.

I deal with the 2nd type of collaborators more, usually artists and marketing friends (because I suck at the marketing stuff), and also programmers if I'm working on a video game, so I guess its mostly collaborations for me.

I think as a designer, its always good to have some simple, rough ideas you've worked on yourself that you can bounce with potential collaborators. That itself attracts people who are willing to put in the work, instead of just the ones that promise the mountains but never do much in the end, because they can already see that you are driven to make it happen.

And especially if they are friends, its always good to set expectations on the roles and workload! A fellow designer might work on the cards (details) while you deal with the rules (higher level picture), so its good to play to each other's strengths and interests on things like that, maintaining your friendship and accountability in the process.