[andreyvit]

I think your example is perfect. What he gave you is not a complete answer to why, because, as you say, he might have been doing a number of other jobs. So why roofs specifically? Add the description of how he inherited his father’s carpenter business but whenever he visited clients, they all had leaky roof, yadayada, and that he now feels happy enough to not look for any further changes, and you get the kind of answer OP is looking for.