My good friend, former colleague, and expert software tester, Lena Houser, recently compiled her take on the relationship between testers and developers, drawing from interviews with developers, including me, on traits they'd like to see in software testers.
Personally, I love testers, and as a developer, it is my mission in life to make their job as easy as possible. I never understood how developers could become defensive when a tester finds a problem. If it doesn't work the way the tester expects, then it probably won't work the way the end user expects, either! The biggest challenge is probably poorly specified requirements - the developer had one way of thinking about it and implemented it that way, while the tester had a different interpretation and wanted it to work another way. As all developers and testers know, getting good requirements is a luxury that few actually get. So it is important that developers and testers work together as a team, rather than as adversaries. It's the only way to produce good software, and it goes a long way to preserving sanity, too.
Lena's post goes over some of the other key qualities in the tester/developer relationship, with ideas from a variety of industry veterans.