Giving to a person in need in a store line

I have heard many stories (and songs) about people standing in line at a store and seeing another person in obvious need and then helping that person. Those stories are touching and move me to want to help and look for situations like that were I could help. But AFAIK I have never been in such a situation. Have you ever been in a situation like that? Are they just rare in the areas where I go? Do I just not notice them? Do I just assume the person really isn't in need? Why do I hear those stories but never seem to get a chance to act out the good deed?