Not necessary. 有的教授一起写了其实心底也分外眼红呢。for example, there a 2 Chinese faculty in the same department, and other two faculty mamber from other two Asian countries, such as India, or Korea. Then the two Chinese have no other option, they two can only co-author. But they two may not have good relationship though. Probably just looks good, while not really good. Co-authorship is just a common interest relationship.