Why are we so against registration?

Because- among other reasons- the National Socialists gave a very good lesson on the matter.
In 1932, Alfred Flatow registered three handguns, as decreed by the harsh gun control laws of the Weimar Republic that were drawn amid the violence and chaos of the aftermath of World War I. The Weimar Republican grew out of the ashes of the Great War. Perceived as an institution imposed by Germany's enemies, Weimar lacked political legitimacy and was vulnerable to insurrection.
...
Gestapo legal adviser Werner Best proposed to execute Jews who were found in possession of firearms. He had both ample precedent and experience to draw upon from the harsh gun laws and their brutal enforcement in the Weimar Republic.

In 1938, in preparation for Kristallnacht (November 9-10, 1938), the Gestapo used the Weimar gun registration records to disarm Jews and focused on Jewish gun owners for deportation to concentration camps. Alfred Flatow fled Germany for the Netherlands, but when the Nazis invaded the Netherlands, in May 1940, Flatow was on the Gestapo's list.

Flatow was arrested and sent to Theresienstadt Concentration Camp, where he died from starvation on December 28, 1942. His cousin Gustav died in the same camp three years later.
Every time I hear some anti-gun/hoplophobic sort sneer "So you think the Nazis are coming for you?" I remember two things:
A lot of Jews didn't believe it could happen to them, not in Germany.
How many fine little progressives have said that people like me should be rounded up and put in camps?

I wonder if they have any idea of the parallels?  Or would care if they did?