The quick answer to this is that agricultural societies allow more people to live together in relatively small areas. Agriculture allows for more food production and that allows people to live in cities and for some people to do things other than farm.
Once you have a situation where not everyone has to farm, you can have some of the proximate causes. You can have political organization and writing because the cities need governments and they need writing to keep track of how much is produced and of what the laws are and such. You can have guns and steel and ships because people can learn how to make those instead of just farming.
You also get the epidemic diseases because those come from people living in close proximity to farm animals.