What makes partnering up successful?

This is slightly off topic (and I hope that it has not been asked/answered recently): I would like to get your opinion on what the expectations towards a partnership are and what success factors you personally see? There are some obvious gains if both view the relationship as favorable but I am sure that there are motives beyond pure exchange of services/skills...

If I take myself, I actually do want to partner up with someone but since I have primarily been programming my own stuff, it is hard to tell how I compare against unknown expectations (hello impostor syndrome). But for those people who are posting here it is more or less obvious that they don't have a matching counterpart in their immediate surrounding either and that made me curious what your personal motivations are for reaching out here.

Besides the first expectations and success from above, here are a few more concrete questions I have in mind on that topic:

1. Are you viewing it as an extension to other networking/recruiting platforms?
2. If so, why IH and not the others?
3. What is your primary motivation? Business or other aspects, e.g. enhancing own skills, friendship, change of perspective, ...?

Please feel free to comment, really curious to read your thoughts!