Being vulnerable is fucking terrifying. It can also be extremely rewarding and beautiful. I'd say that new love is the scary part - old love should feel comfortable
It's having someone completely emerged into the world that you created. All the odd things about you, all the inside jokes, being so comfortable around each other that something as simple as being naked isn't just an idiom for the way you act around them. You're not guarded. The feeling of being in love is so tremendously freeing. You're two different people, your own bodies on this planet we call Earth... and yet, you still find ways to feel like one whenever you're together. It's saying a random thought in a silent room only for your love to look at you confused and for them to tell you to get out of their head. (And a couple hundred times more.) It's watching a silly movie after a hard day and to quote that movie for days to come, turning a bad day into a lovely memory. It's poking fun at each other when one makes a mistake and to motion for them to use their kidneys (whilst tapping your temple). It's picking up on their weird quirks as they turn into yours and vice versa. Or when you get into a disagreement and the way to break the ice to feel like one again is to make a face and call the other person grumpy.
It's also taking a step back and realizing just how much you've grown with that person, and that if for some reason the world said that your time was up, how heartbroken you'd be to be apart from them.