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I don't know what the scientific, chemical explanations are although it'll probably have to do with endorphins and such.
However, I can tell you what psychological and emotional matters influence our state (and why for some of us it is so hard to maintain it).
You see when we are going through this world, we have a bunch of thoughts running through our heads. Most of them are totally unconscious! So, if you don't know why you can;t keep your state, even though you are trying to go out of your way to enhance it, it's because you are not aware of what actually goes on in your mind. It can come from deep inner conflicts coming from past conditioning and so on. It might come from traumas from our childhood. It can get very complicated. But the thing is that every time we are faced with situations that unconsciously remind us of our "wounds" our state lowers. Meaning that our negative thoughts (most of which are highly unconscious) influence our brain chemistry.
Before I discovered this fact, I was desperately running around trying to pump my state and then feeling guilty that I couldn't do it for long enough. Well, I know better now

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I think this post somewhat true. If you read "The Power of Now" by Eckhart Tolle, it teaches you what state is and how to turn it on, in different words, obviously.
He claims (and I have found it to be true in my own practice) that the next stage of humanity, that leads to pure connections with everything and everyone around you, comes from consciousness. This means being completely in the present moment and also being completely aware of the feelings and energy inside yourself. It means stopping mind activity, including fantasies about the future or reliving memories of the past (he describes how to do this in the book. You'll find its harder than it seems).
In my experience, when I feel most in the now, I connect to people on the most genuine level. When I combine feeling in the now with excitement and adrenaline, I'm "in state" and can be a social beast. It's very helpful to understand the principles of the book and be able to meditate and place yourself into the now, state follows easily.
IvanDyn is correct in saying that much of our mental activity comes from unconscious factors. It is also important to note that IT DOESN'T MATTER WHAT THESE FACTORS ARE. At all. Focus your attention completely on the now.