Whereas companies often undergo a friendly merger or acquisition, why don't countries more often do this? Set aside colonialism because I'd like to focus on the case where the threat of violence is not paramount.
I suppose the right incentives aren't in place but is this an intrinsic fact of international relations or democratic politics?
For example, a merger between the US and Mexico seems, naively, like it could be in both nations' interest. The US gets more land and lower-cost labor and Mexico gets access to better governance and knowledge spillover.
Autonomy and culture are big points. Mexicans would be losing a lot of autonomy over to the United States, and their culture would to a large degree be overwritten by the more dominant countries culture.
The only situation where I see this working is where no one country can have total power over the others. Something like the EU, if they move towards federation. But even with the relatively loose coupling the EU has, there is plenty of euroskepticism - both founded and unfounded.
In general, the advantages of mergers are not so obvious, while the costs are enormous. You can get many of the benefits of having one country through mechanisms like free trade agreements, open borders, shared currency zones etc.
This is key in my opinion.