As long as we are asking for examples, a very important recent example comes to mind: East and West Germany. This lengthy discussion I've recovered from the search bar might interest you; I had intended to link the FAQs, but I actually cannot find a "German Unification" section in the FAQs.

I do know a thing or two on German Reunification, but only through the lens of the European integration process. Indeed, the reunification of Germany also fits into a wider narrative of European integration, which has roots streching to the end of the Second World War and might also represent something that you're interested in given your question. I examined the social, intellectual, and political origins of the European Integration process in this older answer which might interest you, as well as this shorter answer.

I'd also suggest looking at the Prussian unification of Germany a century prior, which was a largely diplomatic process (not without elements of coercion, but still more diplomatic than military). You can read about that in this discussion which is featured in the Germany FAQs section. Italian Unification a few decades prior, while contrastable to the German experience in that it was very much characterized by conflict, was also helped along by strong political and social currents which led several Italian states to spontaneously appeal for annexation to the Kingdom of Sardinia without explicit military coercion (although these annexations did typically occur in parallel to wider conflicts). I wrote about the Tuscan political motivations for appealing to annexation which I believe is well-aligned with the spirit of your question.

Other (relatively) peaceful fusions of two sovereign states include the union of England and Scotland, and of Castille and Aragon, although the farther back we go our lines of what constitutes a sovereign state get increasingly blurry, as do the institutions through which they are governed.