I understand why volunteers joined different services (i.e. Army, Navy, Air Force), but how come they joined specific units? I understand why some people would have wanted to join the paras or an artillery unit. But how come someone would join, for example, a Canadian Scottish Regiment over the PPCLI, where they essentially did the same job would have recruiting drives in the same city.
If my question needs clarification please ask!
In some cases, like the Patricias, private and affluent individuals helped raise the regiment or battalion. Hamilton Gault, who at the outbreak of the Great War, was captain in Canada's regular force. He and Lt.-Col. Francis Farquhar raised over $100,000, which was about 2 million dollars in 1914, to equip and train a battalion for overseas service. The Canadian government accepted in August 1914 and shortly thereafter the Canadian Governor-in-General signed the regimental charter. The PPCLI, at least initially, was a prestigious group, often consisting of affluent young men. This would change according to the broader contours of the war, personnel and materiel shortages. In a way, this could be compared Lord Kitchener's Pals battalion, although with some obvious differences.
If you're interested in the regimental history of the PPCLI, be sure to see David Bercuson's regimental history The Patricias: A Century of Service (2013).
During the Second World War, personnel selection was somewhat different given the increasing role of tactical air support, armour, and other trades. In addition, Canada opened two OTCs (Officer Training Centre) in Brockville, Ontario and Gordon Head, British Columbia. These training centres were used to create an officer corps in anticipation of Canada's great role in Italy, France, and Northwest Europe. I would argue, then, that officer selection and leadership was somewhat "democratized" compared to the previous war. However, because of mounting casualties--especially in Sicily, Normandy, and the Scheldt--officers were trained quickly (given a 90-day course) and dropped into units/companies/brigades often without knowing anyone at all.
At any rate, I hope that helps for the Canadian context?