1. Dan met the woman of his dreams, but he wasn’t quite ready to get married. They dated for a while and then went their separate ways. They were ships passing in the night.
2. I came to this city in 1985 and left shortly after I met Robert. Our lives touched only briefly. We were ships passing in the night.

The expression is frequently used to describe a romance that is not meant to be because it is the wrong time or the wrong place.