Sometimes a good collaborator is someone who sees eye to eye with you. Sometimes it is someone who is very different, like Lennon and McCartney (also, their collaboration was often more of a competition---many Lennon/McCartney songs were primarily written by one or the other or with only some collaboration in order to finish the song off---but they often were driven to produce better material when one of them came in with a top notch song idea). Sometimes it is when you have a dedicated lyricist and a dedicated music writer, other times it is good when the lines are blurred. I think it comes down to chemistry, patience and talent to collaborate. But many great collaborations had radically different chemistry.