What do you do if you know you are with the person you want to marry but know it's not quite time to take that step; yet something happens and the other person is ready to leave?

Would you ruin the “surprise” of it and tell them your intentions or just keep it in and not make their decision to leave harder? Would it make sense to try and have someone stay based on what could happen even if you didn’t know it would solve anything? Does it seems a little selfish to let something like that out at that time making things a little harder? But if it’s true, don’t they deserve to know it or is that just a guilt trip?