Most of the time I want to capture nice street scene, something that captured my eye while I am walking and looking around. But from time to time I see two or more older people speaking on the street, or on the bench in the park, and I want to approach to them, listen to them and capture their interaction and their expression.
I don't want to show human injustice (sometimes yes, but not in general) - I want to show that life is beautiful even when you are old, poor, lonely ... I want to show loneliness, but also friendship, and to show beauty in that. Beauty in passing moment and beauty in life - this is what I want to show.
In that case, you'll have to either be daring enough to approach the people you're interested in photographing, talk to them, and ask their permission to make a photograph of them. Or, you learn how to do it by simply taking the picture without asking.
Whatever works for you to convey what you wish to achieve. Try both approaches and see how they work for you. Or maybe there's even a third alternative I haven't thought of.
Either way, I think the choices we make in how we photograph, greatly reflects our personalities. Some people are shy and would not be caught dead asking for permission, while to others that would be a breeze to do and 'What's the big deal?'. Find out what suits you.
To me, it's extremely difficult to muster up the question: "Do you mind if I take your portrait?" Very hard for me. Sometimes I do it, but not often enough.