Advice Without Asking

I notice that a lot of people in my age group (18-25) like to give me their advice on a situation when I don't ask for it. I get very annoyed by it & then tell them thanks but no thanks & then they get upset or defensive that I don't want their advice. Can someone please explain to me why this happens? 
As the person that I am, if I want your advice id ask for it. Just because I'm "complaining" to you doesn't mean that I want some advice on it. I'm just venting .