Density and the same people and ideas AKA homogenity of the population.

Look at Japan and rural Western societies. High trust, but the trust is because they are all pretty much the same people, demographics, beliefs, etc. and the density makes it so they pretty much know each other.

I'm not sure if it's directly related to density, see Japan as a counter example.

Or familiarity. I tend to pick one or two favorite food places where ever I live (always in big cities) and go there till I know them and vice versa (locally owned, not chains) and when I forget all my payment methods they always say to pay next time. (Probably helps to be a self-deprecating white guy in this scenario, of course.)