You don't feel any pressure to go out on dates when you're together—you can sit around doing your own things, from homework to reading to whatever else, and that doesn't bother either of you. You just like being together, even if you're not doing anything at all.
Maybe in the beginning of your relationship, you used to put on makeup and do your hair before hanging out with them.
You know you don't need to impress them, so you're comfortable around them no matter what you look like.
Sometimes you need someone to call you on your crap.
The two of you love a lot of the same things, which makes hanging out together easy and enjoyable.
They are still as handsome/gorgeous/sexy/beautiful/all around attractive to you as the day you met them.
You can be your weird self, do whatever you want to do, say whatever you want to say and not feel self-conscious at all.
You don't dismiss one another's point of view when you're fighting—you actually listen to what the other person has to say instead of waiting for your turn to respond.
Even if it means you end up agreeing to disagree, you never go to bed angry.
Even if other people don't think the things you do together are fun, the two of you do, and that's all that matters.
Though you two definitely know how to have fun together, you also don't always need to be doing something to enjoy each other's company.
Of course they think you're amazing just the way you are—but they also know that you have goals and dreams, and they're rooting for you to achieve them. They believe in you and they know that you can accomplish anything you set your mind to.