Each living being can be conveniently called a 'natural machine'. Humans perhaps come under the category of those natural machines which has the maximum capability to think and act on it's own. Each living and even non-living beings follow some unknown yet existing "Laws of Nature", like we had "Laws of Jungle" in Jungle Book! These laws determine our action/reaction, behavior, and decisions, emotions etc.
Also, evolution has played a major role in making us and this world as we are and as it is today.
Some of these Laws of Nature have been discovered like "Laws of gravity", "Laws of motion" etc., yet much is left to be discovered.
The functioning of our being is governed by laws and things like bio-chemical reactions, metabolism, hormonal secretions, or some other body reactions are what runs our body and gives the reaction to stimuli. This can be directly compared to a machine/android.
Hence, our physical being can be compared to a machine and hence can be simulated. The tricky thing here are "emotions". While I believe that emotions are also a part of our brain and can be simulated, many of my friends don't agree to this point.
Another general belief is that brain and mind are different. I think that brain is a physical structure which enables us to think, make decisions and consciously or unconsciously control our body. Mind is something that is the working brain and also handles our emotions. Now, many other species have brain, but they may not have a mind, which means a brain which can feel and interpret things differently so that changes are made. The mind helps us think differently and make discoveries and improve our lives.
I think this mind works on a randomized model which leaves some scope of unexpectedness and innovation but it still follows the "Laws of Nature". This would also mean that emotions, which are a part of what our mind generates, can also be modeled. I think that's also possible, but we haven't done enough "out of box" thinking to come up with a different and appropriate model for simulating emotions.
Also, what I feel is that each and everything that a living being, especially human (the most complex living being) comprises, are interdependent on each other and hence simulating one thing in isolation would not have complete meaning and our intended purpose might not be met!
For example, if we have to simulate pain, then only simulating pain may not give us correct picture of how human perceives pain. If we add present state of a human mind, it's emotions etc. and the kind of person he/she is (afraid of pain maybe), then maybe we'd get a better simulation.