Spurge

Why

Spurge generally grows in disturbed soil, such as after natural disasters or after humans have come in and removed everything else. It popped up when I stopped weeding because it's native to my area.

Where

Can be found across most of the US, especially the east and midwest.

What

This is probably euphorbia maculata. Euphorbia are really interesting because some of them look like that and some of them look like this -->

But all of them exude a poisonous milky sap when cut so be careful if you're pruning or removing them! Spurge is generally considered a weed because of the sap and lack of doing anything 'pretty', as well as growing well in disturbed environments.