[YuriNiyazov]

This depends on the context of a working relationship. It is certainly true that if your manager hired you and some other guy, it is possible for you to work together effectively with the other guy, even if you seriously disliked him personally.

YC is much more of a "mentor+mentee" relationship than a regular "coworkers on a team" relationship. A mentor relationship is much less effective if there's no personal rapport between participants.

[toyg]

Do university professors call all their students individually before lessons start, just to build a "personal rapport"? No, they build it as they are mentoring. That's how it's done, if it's all about mentoring. Mentoring someone you know in advance and have a personal rapport with is just a form of patronizing.

[kenko]

Also, as others (maybe you, elsewhere?) have pointed out, it would have been a good experiment (!) to take Maciej on despite the initial cold feeling just to see what happened. Maybe it would have worked out! Maybe that would be a lesson about relying on vague gut feels!

[diziet]

Thesis advisors certainly do this, though.