So what would prompt a country to do this? Although the press gang seems to have originated in the thirteenth century, it was the Quota Act of 1795 that increased its occurrence. The Act forced every county in Britain to provide a certain number of recruits according to its population. Though civic authorities offered bounties and even freed criminals and debtors to fill the quota, they still did not have enough men.
But these gangs wouldn't just take anyone. They were looking for the following:
Men between the ages of 18 and 55
Not apprentices or gentlemen
No landsmen. The men must have some experience on board a ship.
Before you get too upset, once these men were made to "volunteer", they were treated like anyone else in the Navy. They were given training and pay and clothing and food. Whether they were happy or not about their new career, we may never know, though I imagine their emotions ran the gambit from furiously miserable to happy to have a job.
And just so you know, America never really picked up this practice at large, although individual states did practice impressment from time to time depending on the local law.