This may turn into a Tuesday post but that's ok

I'm curious, when have you dated someone long enough where if you aren't ready for marriage with them, then you might not like them as much as you think?
One of my friends has been dating the same guy for the last 15 years. She is now 35. She asked him to marry her once and he said he needed more time to think about him. After a year or so had passed she asked why he wanted to wait longer, as it had already been 10 years. He said "I don't know if you're the one yet"
*crickets*
Not "I think we should wait until we're financially ready" or "I think we should wait until our jobs are more secure". No, "I don't know if you're the one yet".
She thinks that's a perfectly fine answer. I think if you've dated someone for 15 years and he doesn't know if you are the one, then you obviously aren't and should be getting out of there while you still can.
thoughts?