It's sad that it's not safe to be so trusting anymore, but we have to be protective of ourselves. I've heard of many situations where girls have had flat tires or other kinds of car trouble along the interstates and end up disappearing or being harmed.
A flat tire in a shopping center, with a kind stranger offering to help shouldn't be threatening -- but you need to learn when to raise your hackles.
And always call on someone else instead.