People are fascinating subjects, providing endless variety for the photographer in search of an interesting picture. No matter how many portraits we see, the human face's seemingly infinite variability of shape, colour and expression keep the subject fresh. It's one thing to get a professional model into a studio where you have control of timing, background and lighting, but street portraits are something else entirely.
First you have to work up the nerve to ask. Not easy at any time, but if the person is is engaged in performance, customer service or even just wearing a silly hat, it can be a bit easier. Perhaps even more challenging are the technical aspects of getting the shot. All the things taken for granted in the studio go out the window in the street. Lighting is rarely ideal, backgrounds can be a distracting nightmare, and you don't have the luxury of taking your time - you are after all imposing yourself on peoples busy lives. However, when it comes off, it can be really rewarding, which provides all the encouragement you need to keep trying.