Your vehicle has lots of doors including that one usually near the back on the vehicle's side.  That's the fuel door, something you use every time you gas up.  These endure hundreds of open-and-close cycles, usually without any problems.  But when they act up, it can be a major inconvenience for you.

When they stick in the "open" position, it can present real dilemma.  You can still pump your gas, but do you just drive around with that flap sticking out the side? What happens if someone steals the gas cap or it gets damaged? What happens if it rains? Yep, it's decision time.

A fuel door that sticks open can be due to a number of factors.  The hinge on the door may have broken, possibly from corrosion or it may have been hit sometime.  Some vehicles have a cable that operates the door and it could be loose.  The latch that holds the door shut could have broken or it, too, could be bent from something hitting it. 

You probably want to take care of this sometime soon since your gas cap is wide open and unprotected when the door doesn't shut.  Plus, it's possible that the door could be torn off completely.  Often a stuck open fuel door can be fixed fairly inexpensively and quickly depending on the type of mechanism your vehicle has. Looks like it's an open and shut case.

Tuffy Grand Rapids - Fuller Ave.
1121 Fuller Ave. NE
Grand Rapids, Michigan 49503
616.458.8091
https://www.tuffyfullerave.com/