Here is another example of when I asked for permission to shoot:

The flower guy

So I saw this gentleman selling flowers on a Saturday market. I saw him working and it occurred to me to tell a story of his work. So I needed to have a couple of shots from him. It would have been difficult to lurk around for a couple of minutes to take the shots I needed, so I went to him and asked him directly if I could take his picture while he works. He was not shocked, nor asked why. He just looked at me and said ok! So I could go around him, and shoot several photos at different angles with different customers, and develop the story that I wanted to tell. So the moral of the story: If you want to take your time with your subject, it’s better to go and ask them directly if you can photograph them. This way, they won’t get annoyed and you will feel more comfortable.