"the best way to avoid [[getting cheated/swindled]] is to give strangers your trust and treat them well. Being good to them brings out their good. If you are on your best behavior, they will be on their best behavior."
As someone who's lived in London for 20 years, if you trust and treat everyone well you will lose all of your money, your watch, and phone. Unfortunately, those looking to cheat you out of something have become extremely common and totally remorseless!
I get that the author is probably referring to locals but these days it's impossible to tell who's a local trying to be kind and who's trying to distract you to reach into your bag.
Maybe I'm being unnecessarily pessimistic about strangers but i guess that's what London does to you haha