Thanks to globalization people have more choice in the environments they live in and change environments with much more ease than in the past. While talking about relationships the focus naturally goes to individuals, it should be recognized that growth in relationships on top of ever changing environments gets much more tricky.

Transport, Communication, Finance, Info networks have all changed drastically from the past.

And the sociologists have been pointing that out for a long time now. Check out

Zygmunt Bauman's Liquid Modernity.

Beck's Normal Chaos of Love

Works of Judith Stacey

Its a complex ever changing world out there. Coevolution requires a bit of stablity in the environments. Focus should not just be on what individuals are doing for each other but on how they shape the environment around them in ways that makes such things more possible.