Jeremiah is fasting with his weeping not merely weeping, and he is envisioning not just taking it on the chin. It is for the purpose of action. He is a priest and a child comparing his world to another world or another time that he knows is possible, very tenderhearted and hot blooded. Chap 2:18 talks about returning to the ways of Egypt, and chapter three introduces the term 'Backsliding'.
We know now through the many available documents discovered in the last few centuries that ancient Egypt is a horror story. All of the things that Jeremiah is lamenting happen regularly in Egypt. People believe in pillaging, war and slavery there. Jeremiah learns that some people are not only returning to the 'Ways' of the Egyptians but are actually going to move there. In chapter four he says people need to be circumcised again in their hearts. (Its like they've never left Egypt and are still slaves there.) He's weeping and fasting, and he's also doing so publicly and gets into a lot of trouble with leadership.
