Public Relations and Journalism are symbiotic because they both have a valuable resource that the other needs. PR has news and information that the news media may not have gotten itself, and the news media has the audience that the PR people need. So they are willing to work together.
It is strained because they both have different goals and intentions, and different ideas about what is important news. As the book says, journalists view news as some sort of change in the status quo, while PR thinks that it is good news when things are going good and steady. That is just one of the few differences that causes the relationships to be strained.

Now, what I really want to say.... This strained relationship is a good thing. They keep each other honest. If newspapers always wrote about everything the PR guys gave them, never questioning or getting the other side, the PR people would get away with whatever they want, and they'd probably DO whatever they wanted. And we'd end up with crap in the news. It keeps the PR people looking for real, honest, and newsworthy things to report.

On the other hand, if PR people weren't always presenting things to the news media, there would be a lot of information and news that the public would miss out on...information that they may have needed. It keeps the news media out of one little bubble, and keeps them looking at the variety of stories surrounding them.

So, frustrating as it may be, the tension and conflict is a good thing... As long as they still work together.