Gas Prices

Same reason any other globally traded commodity's prices change when there is a supply increase or decrease or a demand increase or decrease. 
Some countries do prop up their own domestically produced oil (and currency) for their citizenry but do so at a high cost to that country. Venezuela tried it for years and they went bankrupt. Russia does it as well and indiscriminately charges more for their energy products to some countries than they do others, while subsidising retail prices for their own citizens..except in Chechnya....they have had a running dispute and sometimes civil war with that part of Russia.

Crude oil and it's refined products (gasoline, diesel, nat gas) is bought and sold on the NY Mercantile Exchange here in the US. Whatever a bbl of oil is for any given day at closing, 3:30PM Eastern time, that is what a BBL costs to the refiners. If the price goes up, then all the products made from that BBL goes up as well. Gasoline prices on the retail level is set on a daily basis according to what it will cost to replace what is in the retailer's underground tanks nowadays because most retailers are independent of the refiners. (convenient stores/walmart/COSTCO etc)  Convenient stores sell 80% of the retail fuel in the US and there are somewhere between 115,000-150,000 independent retailers in the country.
 
The biggest majority of those, have to buy NOT from refiners but from bulk providers and few of those retailers are company owned nowadays. If they sell the gasoline in their tanks based on what they bought it for, if price of bulk gasoline on NYME goes up, they would lose their britches when it came time to refill their tanks.

Why does the price often go up overnight?
Because every day around 5:30 Eastern, the bulk wholesalers send out a fax or other electronic notification to all their customers advising them of what it will cost to replenish what they have in their tanks and the retailer adjusts their prices accordingly. This is called the "rack price". The bulk providers base their costs on what it will cost them to replenish their supply from the refiners on any given day, and pass their costs on to the retailer.
There are some retailers that buy directly from the refiners but most do not, as it generally cost more to buy directly from a refiner because that means they sell branded gasoline. 

What determines Rack Price?
The International Association of Securities Commission (most of the different Western nations' market indexes) agreed to use what is known as OPIS to determine rack price. Oil Industry Price Index. OPIS was chosen by the US Securities and Exchange Commission because it was deemed to be the most accurate and objective (fair) analyst of energy costs and prices. OPIS is scrutinized by SEC to ensure they are doing their job fairly and without bias. It is OPIS that sets the daily rack price dependent on a lot of different factors.

IOW, contrary to popular belief by many uninformed people, "Big Oil" can not and does not arbitrarily raise the price of crude oil or retail fuel at their own discretion or on a whim.

It is illegal in the US for retailers to sell retail fuel below costs. Wasn't always the case and it's why gas wars used to be the common.This law was passed because there were too many cases of big retail chains and company owned stations trying to run mom & pop gas stations out of business.  Retailers can Raise their prices any time but most do not, as they would simply loose customers to the stations that held to the line they got from their bulk provider. 

Producers of beef, grains, and any other globally traded commodity also have to deal with global prices and most of those are set by Chicago Mercantile Exchange.