Welcome week at INSEAD

I am born in a city which you would usually call a cold city. When you go outside in places or restaurants, when you go to pubs or discos, it is not so common to meet new people if you are not introduced to.
Now you could object: "it's your approach, it is not the city". Well, you are partly right, I am a rather shy person, but it is not only matter of personality, I can assure, you can ask the same question to extremely extrovert people.
I have always liked the idea of complex systems. That is, systems in which the result is not simply the sum of the parts, but there is an "identity" of the system itself, to which you would give a name or adjectives. This is the case of a city, and this is the case of INSEAD.
What I saw yesterday was something amazing. I was in a big open space of a chateaux. There would have been about 500 people.
It was bout 1 a.m and I was already there from a couple of hours. I was already immersed in speaking with my friends (where the closest friends are the ones that I met 1 week ago!), eventually speaking with P3s (you not-inseader guys have to figure it out what it means), having some coktails, dancing a little bit, having fun.
But then for a moment I stopped, and started to observe what it was happening around me.
And what I saw was amazing: people were interacting with each other as though they knew them from years. Everybody seemed to be the closest friend of everybody else. And all that was incredibly spontaneous. That was real fun. It was a real party atmosphere. And all that was completely natural and straightforward. That was the normal way to act.
You could wonder how all that could happen. Why does this not happen in other situations?
Is a matter of people? Has INSEAD done a pre-selection of the most open and partying people of the world? I am sure that some of us would say yes, but I cannot really imagine these 500 people interacting in the same way in another context.
It was not about the people. Again, the system is more than the sum of its parts.
So, was it about the alcoholic level? On the other hand, the party was free and there was an open bar (thanks Bain!). But again, this was not the first open-bar party to which I went to. And there was no way that the atmosphere could be compared.
Ehy, I got it: It was about P3s. In reality INSEAD artificially created this partying atmosphere 30 years ago, and then it created these two overlapping intakes so that fresh students are "contaminated" by more senior ones.
But wait a moment... I went also to other dinners or parties in which there were only P1s, and the atmosphere was always the same.
So, how could all that happen? How it is possibile that hundreds of people, which had never met before, happen to behave in that amazing way?
And how can it happen each year, for every new intake?
And since I could not find an answer to these questions, I promtly returned to enjoying the INSEAD atmosphere.