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Though I am certain someone could do it, it is hard to dispute the connection between having choices and control over your daily work and overall job satisfaction. The people with more power to govern themselves will typically report that they are more satisfied with their jobs. This is why pairing can play a huge part both with picking who you work with and what you work on.
The difference between a 1 pair project and a 2 pair project is 3 choices vs. no choices in who you work with. You can like everyone on your team and be pleased with their work, but you can only sit side-by-side with one guy for so long. When you go up to two pairs, you suddenly have a choice that wasn’t there before, and it is there in a big way. Tired of Joe? No problem, you and Rachel haven’t paired in a while, so make that happen.
The reason people don’t need “pet projects” is because we get to pick who works on what at the top of the backlog. Yes, I have to do some grunty work, but I also get to side-step some things that I’d rather not do. More importantly, you pick what it is you work on. So now you’re doing something you chose, vs. something that was assigned to you. You as an individual have a personal investment in seeing that task through.
The work place can be filled with all kinds of challenges, and can seem oppressive if you can’t exact any control over it. By pairing and having an option to rotate, you get to pick your work and pick your coworker every single day. And choice is a great way to stay happy at work.