The US created south korea out of thin air at the end of WWII, literally just drawing a line on a map.

Then they both held elections. The south's election was rigged by the US, who used their sway at the UN (the USSR was boycotting at the time and PRC still hadn't been accepted) to get South Korea's puppet state recognized as the gov't of all Korea, including the parts that didn't even have the US's sham elections. As preparation to invade the north, the US purged any non-compliant elements from the gov't (going so far as to put compradors who'd worked for Japan during occupation in high ranking positions) and carried out massacres of elements likely to side with communists (such as rural villages that lead communal lifestyles).

The north saw America was coming for them and the longer they waited, the worse position they'd be in.