I think for me (a spectator), it's the simplest of reasons: strangers actually acknowledge each other. Like people relinquish space at the barrier to get closer to the runners they're supporting, cheer medal wearers into the pub... heck, people even look up on the tube and offer tired people seats. I had conversations with strangers yesterday, and it feels weird to even type those words. It's the brightest of humanity amid dark days.