Why do you need to know?

Sometimes I find its really hard to be a girl. Men always say the hardest thing to do is to ask a girl out.But I say the hardest thing to do is to tell them no. You feel like a heartless bitch trying to let them down gently. And sometimes, its like they just don't want to hear it.Recently i've been found defending myself over saying no. I've had to give validation.I believe that when you say no that should be the end of it,no validation required.Why do you need another reason.No should mean no;Not maybe.
Being 17, I may be a little naive in saying this but when did no become an option? Its seems like no is not enough anymore. You can't just say no.Then your always faced with the "why"question.That dreadful question you don't have an answer to. Men only hear what they want to hear.Some how in their minds,no means maybe,they could still convince me otherwise. Think about it....really think about it... Whens the last time you told a guy no; without an explanation or reasoning and they just accepted that.
When did "because I don't want to" not be good enough?