Why the cabin is so dry and your ears keep clicking

The air inside comes from outside, and outside there is far less of it and almost no moisture at all.

At cruise the air outside is extremely thin and extremely cold, and cold air holds very little water. Since the cabin is supplied from that outside air, warmed and pressurised before it reaches you, the moisture simply is not there to begin with. Human breath and the odd cup of tea add a little back, and no more. That is the entire reason for the dry eyes, the dry throat, the thirst and the strangely muted taste of food. It is a consequence of where you are, not a failure of the aeroplane, and drinking water genuinely helps.

Pressurisation is the other half of that story. Because the outside air is too thin to breathe comfortably, the cabin is sealed and kept at a pressure much closer to the ground than to the surroundings. It is not held exactly at sea level, partly because the structure would have to be far heavier for no real benefit, so the cabin gently climbs and descends with the flight while staying well within the range in which people are comfortable and alert. The change is gradual and continuous, which is why it is easy to miss until your ears mention it.

Ears click because the small air space behind each eardrum needs to keep pace with the cabin. Its only connection to the outside world is a narrow tube that opens when you swallow, yawn or move your jaw. On descent the cabin pressure rises faster than a lazy tube will equalise, so pressure builds against the eardrum until it opens with a click. Chewing, swallowing and yawning simply prompt it more often. A head cold makes the tube sluggish and the whole process uncomfortable, which is the one genuinely medical part of an otherwise mundane sensation.