it's hard to try and explain. Ive had this discussion with people before and I think you either just 'know' you're with the right person ; and if you don't know it you're probably not with the one.
I could write pages on here about how my husband is my best friend and how we can still reduce each other to helpless giggles after 16 years together, or the fact I still fancy the pants off him; or a thousand other little things - but none of it would come close to really explaining how I feel about him.
We're just us, and it works, and I hope it never changes.
Not the most helpful answer in the world, is it!