I can't imagine proposing to someone you've been with and them turning you down.
Or I know some who not necessary say no but don't think it's right timing more like a not right now and the guy takes that has rejection and ends the relationship. I know traditionally when you get engaged you get married within 1-2 years. But now a days you have people that gets engaged and stay engage for years. If you know you would say yes further down the road why not just say yes.
A lady I work with technically they are engaged although she refers to him as her husband. But she told me she doesn't want to get married again. Seems odd to get and stay engaged without the intent of getting married. But then again past a certain age and time length together fiancee sounds a lot better than boyfriend/girlfriend.