First of all,hello everybody,I am new in this cummunity

My bearded dragon is about 7-8 months old,and I had him since september 2010.He is very healthy,he's being well fed and good taken care of.
The only problem I got,he is aggressive.Actually,he won't bite,as far as I've seen,but everytime I try to get my hands on him,he get's all puffed up and starts hissing.This is only when I gently rub his head with my finger.If I try to rub his back,he slowly starts to wave his tail,until he suddenly goes away.When I try to pick him up,he always runs away,and if I manage to grab him,he is always squirming like crazy.
He's been like this since I got him.
What should I do?