When I first started smoking I wasn't paranoid at all. I even walked into work right after smoking a blunt (not to work though, just had to grab something) and wasn't even phased by it. Abotu 6 mos. later I slowly started to get paranoid of leaving my house while high. I eventually realized why: I was afraid to get in trouble so I kept slowly finding more things to be afraid of.

I eventually thought about all my fears and over the next 3 or 4 months I slowly tried to conquer them. Now I rarely am sketched out unless I get really baked. Your subconcious is extremely powerful, I gave myself reasons to be scared (many of them invalid) and eventually my mind automatically made me afraid of them if that makes sense.