If the main purpose of guns is self defense, why do we need regulations and rules regarding them?

Because not everyone is in desperate need of one or will use them responsibly.

If we gave out guns to anyone who wanted one society would crumble.

Epi-pens are of course not as lethal as guns but if you use one and don't need it you're going to damage yourself. Adrenaline is great if you need to outrun an assailant or wild animal or need your heart to start beating but too much of it will damage your heart and muscles.

When they talk about Mothers lifting cars off their children in an Adrenaline rush those mothers often also end up needing medical attention because they tore their muscles to shreds lifting that car, not noticing because Adrenaline can turn off your pain receptors.

People who are/were "Adrenaline junkies" doing high intensity sports or stunts tend to have a larger chance of heart issues because they're overworking their hearts everytime they get their high meaning they go bad earlier.