It's so difficult to articulate it properly because it's mostly a feeling.
Some tangible things though were when I knew that we had the exact same value system, both wanted the same things from life, were on the same timetable, etc.
But it was mostly when I knew that I could never be with another person in this way. Sure, I can go on, have other relationships but I know that I will never have emotions this strong for another man as long as I live. No way was I going to let this pass me by.