The "gateway drug" of "gun rights"
For me, it was a combination of gun rights, property rights, and love for the environment.
But, back to gun rights. What I see happen frequently is that (intelligent) people who are passionate about gun rights come to see that it depends on all the other rights. Liberty can't be dissected.
The biggest dangers to gun rights have been the War on Politically Incorrect Drugs and The War on a Scary Emotion... I mean "Terror".
What people see is that supporting these nonsensical violations of life, liberty, and the pursuit of happiness only whittles away on the liberty to own and to carry weapons, including ANY weapon owned (or deployed) by any employee of The State. (Yes, I mean "nukes" as well. If you shouldn't have one in your garage, then the US Government shouldn't have any either. And, if everyone had to pay for their own nukes out of their own pockets, I doubt you'd see anyone having them anyway. End tangent.)
Concern for the fundamental human right to own and to carry weapons- and particularly the collectivist drive to demonize that right- serves to wake up a lot of people. "If government extremists are wrong on this, what else might they be wrong about?" And thinking people will then look into that question and discover that the government extremists are wrong about everything. They'll be addicted to liberty from that moment on.