whoever said "you just know"...it's so true but so hard to explain.
for me it began with a feeling of comfort (kinda like what
Alaska Catdog said about "I could talk to you forever")...there is no fear or apprehension that you will say or do the wrong thing.
Later it was a confidence that this person felt the same way. There was no doubt in my mind that he loved me...
I also agree with what
TimeforMe said...when you feel so "lost" without that person, not a "dependency" so much as a "completion". It's not that you COULDN'T live without them, it's when you realize that you don't want to...
Life just seems so much fuller because you know there is someone there who will share all the little moments of the day with you...no matter if they are good or bad.
It is really hard to explain...but when it happens, it's magical!