How did you know...?

My boyfriend and I know that we love each other, love being around each other, but it frustrates us that we can't pinpoint why. We started discussing the reasons why people end up together, and how they KNOW they are with the person they're supposed to be with. So, I need some input. If you are in a relationship you love and enjoy, how do you just... know? Is there any clue or feeling? Thanks for any help!