If you look at public relations and journalism there is one thing in common... the public. Both fields are trying to inform the public in some way or another. It just so happens that PR people are informing from their client's perspective while journalists are supposed to be unbiased or "watchdogs" for the public. In truth each could not successfully complete their jobs without one another.
Journalists have to cover so many different beats that sometimes things are passed up. That is where PR people jump in. They help inform journalists when certain events are happening for their companies or organizations. That way if the press wants to attend they can.
The real conflict between PR and journalists comes in how and what the public learns. Journalists want all of the story while PR may want to spin it to the advantage of their client. It just comes down to where a person's loyalties are. PRp people have a loyalty to their clients while journalists have it to the public. Although PR may want to inform the public, they are not the first priority. The loyalty difference creates tension between the two. This tension will probably never disappear but journalists and PR people will always have to work together. They may not want to but they need to to successfully complete their jobs. This need for one another will keep journalists and PR people cooperating while they may hate one another.