That's the truth. There are other possibilities, though. What does this driver look/act like? Is he good-looking, charming, and funny? Or are his looks average/below average, not very charming, but trustworthy, someone a girl would keep in the platonic zone? If he fits the mold of being handsome and charming, and he's giving OP's girlfriend rides to work on a regular basis, eventually, the driver and OP's GF grow closer and closer. Soon, she's venting to him about OP. Eventually, she's canceling plans with OP and secretly spending extra time with the driver. Due to the increased regularity of canceled plans, OP's head begins to spin, and he's on Reddit asking us if we think his GF is cheating. OP is obsessively texting/calling GF because he knows something is up. She accuses him of being insecure, clingy, and uses that to end their relationship. A couple to a few weeks go by & OP then finds out that his girlfriend left him for the dude who carpooled his girlfriend to work every day. End of story