Let's see... motivations for signing up for the Second World in 1939, that I've heard of from books and contemporary accounts (in no particular order of precedence):
1. Feeling of duty/obligation to fight with/for the Empire
2. Desire to restore liberty to nations conquered by the Axis.
3. Excitement at prospect of adventure vs. boredom at home.
4. Peer pressure (everyone else doing it)
5. Eagerness to experience what their fathers had experienced in the First World War / prove themselves.
6. General hatred of Fascism/Racism embraced by Axis
Those are the reasons that come to mind, of those, number 1 is the one I recall reading most often in contemporary accounts, followed by 2. and 3. Often in interviews with veterans, you hear the wording: "It was what you did - what was expected of you." or "I don't really know... it was/felt like the right thing to do."