For a time Poland and Lithuania were one country, the Polish-Lithuanian Commonwealth. However, over the course of the 17th and 18th centuries, it was gradually subsumed by Prussia, Austria, and Russia. When both countries received independence after the war, there were significant border feuds between them. For instance, Polish troops basically walked into Wilno/Vilnus (the modern day capital of Lithuania) and seized the city, declaring "independence" from Lithuanian and then deciding to join Poland. The reason? Because the head of Poland, Pilsudski, came from Vilnus/Wilno, and wanted it to be a part of Poland. Needless to say these border disputes led to significant resentment by Lithuanians, on top of a fear that this was merely the first step towards a Polish invasion. As for Vilnus, the city was given back to Lithuania after the 1939 invasion of Poland by Nazi Germany and the USSR. There was a lot of bad blood as a result, and Polish resistance clashed with occupied Lithuanians throughout World War II, further worsening relations. To this day, there is still tension over the border dispute and the subsequent conflicts in WWII.

Sources:

Davies, White Eagle, Red Star

Bor-Komorowski, Secret Army