It seems that if I stop for any reason, someone will ask. I may have stopped just to eat something, or check my map, or just for a rest. One time when my wife and I were first getting back into cycling, she had a problem with her derailleur and someone stop and helped get it adjusted.

For cars, there's AAA and tow services, for bikes you're pretty much on your own unless another cyclist is willing to help you out. I don't know of any bike shops that offer the equivalent of a tow service, though when I am out riding I always make a mental note of any bike shop I pass, in the event I should find myself in need of something I am not carrying with me (this hasn't actually happened yet).