Prices of nearly any commodity in the modern economy, which is often described as a "free market," varies according to a model called "offer vs. demand," i.e., how much of the commodity exists or is produced versus how much is demanded or used or consumed.
In the case of gas, the amount of gas that exists in the world is more or less finite. Gas is produced by drilling for it in reservoirs that contain it. The more we drill, the less gas there remains and the less we can produce.
Also, some countries are developing. When a big country, like China, starts to develop fast, it tends to consume rapidly increasing amounts of resources, such as coal, oil, gas etc. for energy.
All these are apparently the main reasons for the steady increase in gas prices over the recent decades.