Well I'm just seventeen so I don't have this whole lifetime of experience but I think I could say that I know enough.
It's simple really though we are known to make it a difficult thing to realize. You'd see it quite often if you watch enough movies with a love story and it happens, believe it or not, more often in life. We all can't be Allie from The Notebook, and have 365 heartfelt letters to seal that, "I love only him" deal or fall for a girl so hard that she changes your ways like Landon in A Walk To Remember. Well yeah I mean, we may not have the cleverly contructed musical score to match the scene but it's there. What you have to realize about the moment is it's not gonna happen like a scripted Nicholas Sparks novel, it really just happens. It just hits you. You suddenly realize whether you have the dramatic close up or not that "I love __"
Okay so now you've grazed the surface, the difficult part, finding the right moment to say it :P