The #1 thing I would say is that it's comfortable. For all the craziness, we've put each other through, and the shit that continues to happen (I'll spare you a recap of the injuries and hospitalizations) there's just a sense of being safe and secure that the other one will always be there, and all will be forgiven, if necessary.
I think the one thing that marks us as long term couple is the shared (if faltering) institutional memory. We have a history together. We have stories. We have the movies seen, the trips taken, the in-jokes, the tragedies, etc. They're all things we experienced together and when people talk about "ties that bind" I think that's one of the main things.
One reason I can't imagine being in a relationship with anyone else or having to start over, is how could I be with someone who hasn't been through all those things with me?
I fully understand that life is suffering and everything I love will be taken from me. (Living with a neophyte Buddhist is so much fun.) However, I also cling to the illusion that none of that is true and that we will be "together forever" simply because I feel like we already have.