Prices vary between countries because of different transportation costs, conditions such as wages, and fluctuations in the exchange rate.
The primary reason for the price variation from country to country is driven by the difference in the level of tax levied on the fuel by the nation’s government.
Diesel is often cheaper than petrol due to the lower level of excise duty applied to diesel compared to petrol, and again the levels of tax levied by governments in different countries on petrol and diesel can vary which can greatly affect the price you pay at the pump.