In my experience this is a matter of entropy: if you have a partner at home, you are guaranteed support, so you neglect your friends. Your friends then need t

In my experience this is a matter of entropy: if you have a partner at home, you are guaranteed support, so you neglect your friends. Your friends then need to either pick up the slack, or the group fails. So they pick up the slack until they have other options (like a partner at home). Then everyone stops bothering. Then people act surprised. I've seen this happen multiple times now. No one makes an explicit decision to kill the friendship. They just stop maintaining it. Another way to look at this is the tragedy of the commons or "privatising the profit, socialising the losses". It's the same as a lot of social things, from public transport to pensions, from relationships to workers unions. People often end up with suboptimal outcomes by optimising for themselves blindly. And no one person can stop it.