Questions about marraige!?

My boyfriend and I were talking the other day after coming home from my best friends wedding. Of course I was the maid of honor and totally in the love mood. He asked me, how do you know you want to marry me? How do you know you even want to be married? I didnt answer him right back, and he said exactly...you dont know. He said when we both have these answers answered is when we can start thinking about it. He wants to be sure. He said he doesnt want to be another statistic. So my question to yall is....When do you know? I think when you know, you know. We already live with eachother and i enjoy it. I like being with him. I would love to spend the rest of my life with him. But that answer isnt enough for him. He needs more urance. When do you know?