My 8 year old is constantly asking me, why is that person not being safe? or why did that person do that?, why did my friend say that?... I try to tell her my best guess or I say I can't read other people minds. Neither is getting her to stop asking these types of questions. Any suggestions on this?
Encourage her! Ask her, What do you think is the reason?
I think it's a sign of excellent perception that she realizes that other people do things that she would not do, or that she would do differently. She is trying to understand her world, and she wants your help to make her able to "read others' minds" (make her understand their way of thought).
Let her think aloud. Let her explain to you why she's puzzled. Together you can invent some explanations and discuss why explanation X is silly and explanation Y sounds plausible. It will give you a window into her mind.
Some of it is normal exploring of boundaries: if you tell her something is wrong/dangerous, she wants to know why other people are doing it. In other words, why don't they respect the same boundaries that you have set for her? (My kids especially do this when they see someone smoking!)
Answer the questions to the best of your ability. Model empathy. Guess what the people are thinking. Explain why they might be doing something that is against your family's values. Use it as a teachable moment. Reflect the question back at your daughter. Get her to guess first and compare your guesses.
It is only through experience and practice that a person can understand the mind of others. Help your daughter gain those experiences and chances to practice and she will begin to ask those questions less often. When she's 13 or so. ;)
I suppose this is an anti-answer. Sorry. I suggest you encourage those types of questions as long as possible to avoid diminishing her curiosity. When she is curious enough to ask a question, avoid sending her a signal that will make her feel she is wrong to be curious.
What I have done with Alice is to ask her question back to her, "I don't know what do you think?" and see what she says. She often has a thought on the matter and I can give her feedback about her thoughts this way.
I also try to insert ideas about different people think in different ways and if it is about a dangerous activity I'll often said, perhaps they've never thought about it or didn't have some one to help them know differently, but then she started "teaching" others when she saw certain things, so now I've kept it to speaking about different kinds of values and different levels of risk-taking willingness.