Supply and demand. Summertime prices tend to go up because more people are traveling, so the demand is greater. Harsh winters also cause gas prices to go up because both heating oil and gas are made from the same crude. So, if winters are harsh, more heating oil and less gas is made from the crude causing the supply of gas to be short.
@hiuufji and @mmerlinn have defined the "global" dynamics of retail gasoline pricing. However, there are also local issues that affect price. For example, I live in Rhode Island but am right next to the Massachusetts border. RI has higher gasoline taxes, so most days, I will save about $.15 per gallon by buying it in MA. Further, there are several gas stations in my town in RI. The one right off a highway exit ramp charges about $.25 per gallon more than a gas station a mile away in the same town. Additionally, there are two gas stations right next to each other in another section of town. Due to their competitive situation, they often have the lowest gas prices in town.
You came to an internet marketing forum to ask that? You know natural gas isn't a digital product, right?