I think it's the element of chance. You could be sitting next to anyone on public transit. And when love starts there, it's special because it's not in a typical dating setting. It's among the hustle and bustle of daily commuters. There's no pretense, just two people who happen to cross paths and connect while traveling from one place to another. This makes public transit love stories different from the ones that start in more planned social settings.
The shared experience of the journey. On public transit, you're in a confined space with others. In a public transit love story, the couple shares that small world, the stops, the movement, and it gives a unique backdrop to their relationship. They might be going to different destinations in life but for that time on the bus or train, they are on the same path. And also, it often starts from the simplest of interactions, like sharing a pole to hold or a glance during a crowded ride.
The spontaneity. People don't expect to find love on public transit. It just happens out of the blue, like two strangers' paths crossing in an unexpected way.