Most harm, is in fact, done by relatives or friends. Strangers have no motive to harm in the first place, especially if you are not that famous.

Still though, often the reason I don't socialize with strangers:
1. They are moving, so I can assume them to be busy, especailly since on the streets they often are there for the purpose of going somewhere.
2. If they don't speak English...
3. They might be talking to someone else.
4. It is hard to go deep to a total enigma.