I'm probably going to regret this, but here goes.
We get on horribly overcrowded tubes most days of our lives to get to work. people are stressed by the close proximity to people they don't know, that they are hurtling through dark tunnels a long way underground, could get stuck in the tunnels at any point, could be late for work, could get blown up, so basically try to block everyone else out. In order for us to be able to do that effectively, everyone else has to more or less do the same. Sit quietly, don't make eye contact, and get through it, till you squeeze off at the other end.
Now anyone on that carriage who is doing anything unusual, clipping nails, humming, lounging over you, snogging, sniffing, snoring, sneezing, humming, somehow stresses most people out. I can't think anyone cares if you slap on a bit of lipstick, or something quick, but if you start an elaborate beauty routine then it just seems bizarre.
Trains, buses, cars don't seem to have the same effect as you can look out the window and daydream, but being on a rushhour tube just seems to make everything matter more.