Consciousness is a key part of human life. You're not just a machine consisting of matter following the laws of physics; you also experience things. This gets taken for granted, but it actually seems inexplicable. It seems like the core mystery about this universe. It's the reason I'm not an atheist.
The problem is quite simple. We know there's consciousness because we experience it. Science tells us about the nature of matter, energy and physical laws. We know nothing about the connection between consciousness and physics.
I can think of many unanswered questions about human consciousness. Why does a person have a point of view? How is continuity maintained as atoms and even cells are replaced? Why is consciousness linked to the brain only? Why does consciousness stay linked to one particular brain and body? Why is there only one consciousness linked to one brain and body? How does consciousness arise as a new person is formed? What happens to consciousness when a person dies?
Then there's also the question of what is conscious. I know I am conscious, but I can't even prove that other people are conscious. There seems to be no way to prove or disprove that something is conscious. For example, you cannot prove that a rock isn't conscious. However, if it was conscious, what would it be conscious of? Humans have structures which take input, process information and give outputs. Rocks don't have the same sort of thing. What about a computer? It can certainly have capable structures for input, information processing and output. However, the nature of information processing is typically quite different from humans. For example, one cannot really say a computer sees if it takes information from a webcam and compresses and records video. What about a single cell, what could its consciousness be like?
Then there's the question of how consciousness arises. Is an egg cell conscious? Is a sperm conscious? If they were, how did that arise? They come from a bodies which were aware of one consciousness relating to their brains. What happens when the two merge? How is the result conscious while there aren't even any nerve cells in it? You could also ask the question about physical objects. When a stone is quarried or when a brick is made, how can that get its own consciousness? What about if you break a stone in two? What about when it's put into a building?
The question of what happens when a person dies seems to be the only question that's routinely considered. However, it seems like all the effort put into answering this question just leads to myths and theories, and there are no real answers.
Another issue relating to consciousness is free will. It's often taken for granted, but it's actually quite paradoxical. A human is a bunch of matter and energy. There are various physical laws which explain how matter and energy functions. Where is the free will there? It seems like free will could only exist if something else was influencing this bunch of matter and energy. It wouldn't necessarily have to break the laws of physics; maybe it could just affect probability. So far, it doesn't seem like there's any evidence of something like this happening. (Free will is paradoxical even if it does this, but that's probably outside the scope of this post.)
So, there are a lot of questions about consciousness, and there aren't really any answers. Many people claim to give answers. Some talk about myths, some talk about information processing, some invent new myths, and so on. It doesn't seem like there's any credible and sufficient answer.
I have some ideas about how consciousness might work. The brain might just be a sort of antenna, connecting with something which we cannot see. Alternatively, maybe this reality is made of consciousness and matter is only something secondary, like a fractal decoration. This second theory seems more appealing but I can't say I know enough to conclude that it is true. All I really know is that I don't know and that I currently cannot understand this universe.