On the Gemba Panta Rei blog post, Jon Miller says,
" 20% celebration, 80% reflection
In order to do kaizen right you have to celebrate your victories over waste. You need to make it fun. Beware not to let this get out of hand, or start doing kaizen presentations and celebrations for the sake of the free pizza. Take time at the end of every kaizen to ask "what worked and why?" and also "what can we do better?" If you're not too giddy from the celebration, try to convince others that now things are actually worse than ever."
I like this idea that a retrospective, whether at the end of an iteration, release, milestone or project, should partner with time for celebrating the stories completed, features released, and successful experiments or demos. Even if the increment felt tough because of conflicts or challenges, we can celebrate getting to a place of closing that part (issue "I survived the 2nd Foo release" bumpersticker or T-shirts) and finding ourselves on the doorstep of a new opportunity to choose an experiment to improve methods, practices, teamwork, processes, even more.