More than that - when my now fiancee proposed to me I was completely surprised by it, and literally couldn't process the question for a good couple of minutes. Thankfully she decided to leave me to it for that time and by the time she got back I accepted, but it occurred to me that a proposal is itself the only time you're asked to make a life-changing decision by surprise, and answer it there and then without being able to spend any time considering what the correct decision is.