Ok, some of you are asking some good questions and I acknowledge that I may not yet have all the answers.
Having said that, here is my reasoning for why there is similar shared experience by separate consciousnesses (you and me) within the physical world -
Without language to differentiate between 'things' (ie naming of experiences), there is just one single whole undifferentiated consciousness (the 'unity' of eastern philosophy). This is reflected by individuals experiencing a common objective (ie shared) 'world'. However, as individuals (by use of language to designate a separate self) we also strive for a personal and unique experience. This is reflected by our personal subjective experiences (emotions, feelings, desires, will, thought etc).
So what, exactly do you mean by an 'undifferentiated consciousness' and what might it be? How do individuals interact with it? Is it supposed to be, in effect, a god of some sort.
But why (you ask) should we choose the seemingly complicated and abstruse consciousness paradigm rather than the more obvious physical sense based reality ?
1. Because physical reality can be doubted (eg a dream) but the doubter's consciousness cannot be doubted.
Most people, I think, can easily recognise the dream from what they would call reality. I don't think most of us would have a problem with this. As such it rather weakens this idea that we can doubt reality - I really don't think people do.
2. Because we know how consciousness can produce sense perception (eg a dream) but we do not know whether or not matter can produce consciousness.
Well, if we know that consciousness can produce sense perception, then we get back to the problem of what this consciousness actually is. It can't be based on the brain but would have to be separate from it yet appears to stop working when the brain does. Consciousness seems to be tied up with the brain (because it seems to stop when the brain does) so that we would need to explain what the connection is precisely.
3. Because the physical world is subject to cause and effect which means that the first cause (which itself had no cause) is not part of the physical world.
4. Because consciousness is readily associated with perception, emotions, thoughts, feelings, ideas, will, desire etc - but matter is not.
So? Rocks and cars don't produce consciousness? Of course not! but so what? This is not an argument for anything. Most of us on this forum would say the consciousness is an emergent quality of the brain (a thing of matter) and so that the things you list do emerge from matter.
To conclude, Occam and his sharp razor. That the brain produces consciousness requires one less item that your explanation as you have to posit an 'undifferentiated consciousness' in order to explain what we all perceive. Occam would slice that off!