A well regulated Militia, being necessary to the security of a free State...

It's right there in the text. It's worth pointing out, the second amendment is unique in the fact that it's the only law in the entire US Constitution where the justification is provided directly in the text. The reasoning and justification for the second amendment were considered that important to the people who wrote it.

It's also worth pointing out, in the parlance of the time, "well regulated" simply meant "working correctly". A "well regulated" clock, for example, is one that keeps accurate time.

The founding fathers warned against standing armies because they're abused by tyrants. They envisioned, in the event the US was invaded, the response would consist of locals, armed with whatever they could procure, organizing to repel the invading force. This was in the days when private citizens owned muskets, gatling guns, cannons, and warships.

People were also expected to make and practice these invasion response plans, like fire drills, but this was never codified because they never thought they would need to explain that to people; it was simply understood to be a requirement that came with the responsibility.

This made sense in a lot of ways. People who know each other will readily cooperate. Locals will be familiar with the local geography, which was a big advantage in the days before accurate maps or quick communication.

Since then, things have changed. A lot. But the amendment has remained. I still think it's important to enshrine the people's right to self-preservation and self-defense. But I also don't feel like typing a 10K word essay with my thumbs today.