The WikiHow page How to Touch Nettles Without Stinging Yourself claims:
Although experienced nettle harvesters often go bare-handed, this isn't just because they know how to avoid stings. Many of them have become resistant to the stings over years of exposure, while others enjoy the sensation.
This is one of my childhood mysteries which I was never able to fully explain - my neighbour could pick stinging nettles without apparent pain.
Does common contact with stinging nettles confer a resistance to the sting, and thus avoid pain?