Happens all the time. I see the potential for a shot on the street then pause. What if the person reacts badly? Is it really worth it? Am I intruding? Maybe it's not an ideal shot because of this element or this light. Other opportunities will come along. The lens I'm using isn't ideal anyway.
The questions and doubts immediately surface as my brain (some would call it the 'fight or flight' monkey brain that ensured our evolutionary survival) looks for reasons not to make the photograph, to find an easier or safer option instead. Stay comfortable, don't get involved, walk on by.
It doesn't happen for every photograph, but usually every time I go out to shoot on the streets it'll happen sometime. For a while, at least until I find a rhythm, a peace with what I'm doing, the confidence that I belong there and it's what I'm meant to do. Then I become content, content to wait, observe and make the images that come to mind without the questions and doubt. The monkey brain has given up for the day.
Knowing that the hesitation is going to happen somehow makes it a little easier though. It can be anticipated and planned for. Determine to make the image anyway, seek out the fear, be bold. Or move on but don't stress about it. There will always be some missed images, and that's ok. Much as street photographers like to claim that they're 'documenting', that's rarely the case.
Finding out what we care about most though, care enough to do whatever it takes, enough to look foolish, even to fail... that's tough.
Like Steve said though, Here's to the crazy ones.