Just as the title says, why did the 2 nations with compeletly different ethnic makeup, culture, language etc. form a union, i.e. Polish-Lithuanian commonwealth? Even if they did so for xyz reasons at the time and circumstance, why did it last so long, to only be dissolved by foreign powers rather than internal tensions?
To answer this question we have to go back to 1385. Back then Poland suffered a succesion crisis. Casimir III the Great died without a male heir to the throne, as did his brother-in-law Louis I of Hungary, who was a king of Poland between 1370-82. His daughter Jadwiga became a king of Poland. She was promised to Wilhelm Habsburg, but the engagement was broken. Instead Poland and Lithuania formed an Union in Krewo, caused by the danger from mutual enemy of those 2 countries- Teutonic Order which Lithuania struggled to fight against for more than a hundred years. Thanks to this union lithuanian prince Jogaila adn Jadwiga were angaged,Lithuania was baptized, and some historians say it became a province of Poland for a short time and than became polsih feif ( Feliks Koneczny), others say Lithuania became polsih fief at the begging (Oskar Halicki), either way eventually they became two separate countries with personal union (they had one king). The countries were ruled by Jaggielon dynasty for 200 years , but Sigismund II Augustus died without a hear in 1572. Before his death he decided to ensure himself that succesion to the throne after his death is safe. So he decided to change the structres of his realm. Poland and Lithuania became a single country and also became an elective monarchy. Now It's more of my opinion than the facts. It lasted so long, because for a long time it made both countries stronger together, they had one currency, but two treasuries, they also had two armies, but in tough times they obviously were helping each other. It's just that the idea of elective monarchy was really bad. Foreign countries started to support candidates, and with time they were doing more and more to make sure their candidate will become a king. That lead to civil wars and humiliating last election of polish king ( basically the place where election took place was completly surrounded by russian troops to make sure who will win the voting). Poles and Lithuanians got their warning with their first elected king Henry III, who ran away from Poland, when the chance of him becoming the king of France became real., and did nothing about it.
There are some erroneous suppositions in your question.
Poland and Lithuania had simillar ethnic makeup - Slavs (Estern and Western). Why? Because however Grand Duchy of Lithuania was founded by Lithuanians, who are Baltic ethnic group, but after conquest of Rutheanian principalities (mostly by marriages) after end of Mongol reign most of the people in this country were ancestors of today Bellarusian and Ukrainans - who are Slavs like Poles. The official language of Lithuania was ruthenian, plenty of people there were Ortodox (even before baptism of Lithuania), and ruthenian nobility were included in the Lithuanian ruling class. Ruling class of Lithuania quickly Ruthenized.
language in both countries - old polish and old ruthenian. Simillar to each other. Both slavic languages.
there were differences - mostly in religion - Poles were Catholics, Lithuanians were pogans, and after baptism from Poland - Catholics, but most of Ruthenians were Ortodox. But Poland was tolerant country - it allowed Jews and Ormians to live there in XII-XIII century, and in XV century - Tatar muslims too, so Ortodox were no problem.