At first it's like an emotional endorphin high whenever you're around them. It's something else.
Then the high gets less and less potent as time goes on, as you begin to argue, as you spend more time around them, etc. But you can still draw on it, and it still feels good. And it starts to get combined/ replaced with a deeper trust and sense of building a future, that isn't as emotionally crazy, but is more fulfilling.