It's caused by something you're wearing. Man-made fibres attract electrical charges much more than natural fibres like cotton or wool.
I get more electric shocks from door handles when I wear my mixed-fibre woolly sweater.
Try rubber insert soles in your shoes, that should solve the problem.
*sings* Ahm a sole MAN, di dut dur urrr, di dut dururrrrrr...
Or just don't touch anything.