My old car had the tank on the left-hand side, that is the driver's side of the car. I liked it, it made sense to me. But when I got my new car last month I suddenly had a right hand car. It was strange. At first I wasn't sure that I liked it. So I started to research.

Researching this I found a few candidate explanations:

The filler cap is on the opposite side to the exhaust pipe. Makes sense that it would reduce the risk of igniting spilled fuel, and it might also be mechanically easier to build. As for which side the exhaust is on, that will be influenced by the engine mounting especially for a transverse engine.

If there's a lever in the car on the driver's side to release the filler cap, putting the filler cap on the driver's side too is mechanically simpler. But my own car goes against this, so left hand drive, lever on the floor just next to the driver's door, right hand side filler cap.

Probably the car designers just put it where it's easiest, considering the rest of the vehicle. Car manufacturers decide this based on a few things, where static builds (in some cars this is more likely on the drivers side due to materials), ease of use (how close to the driver it is), stopping safety (if stopped on the side of the road so the cap is not in the road, this does only apply to cars based on origin country)... but primarily it is based around where the tank is in the design.

Things like which side the driver sits, a transverse-mounted engine, and certain rear lights all create asymmetries in car design.