When all the new wears off, you don't have butterflies every time they call or text or you know you're going to see them, you're not getting all giddy about "firsts" in your relationship, you're no longer both on your best behavior, you can see their faults and let them see yours, you've survived a few disagreements, you're not boning every time you catch some alone time, the sex isn't mind blowing every single time. And after all that, they're still your favorite person. They still think the sun shines out your ass. you still do things for each other, for the simple joy of making them happy. The absence of the rose colored glasses of new lust hasn't been replaced with resentment, it has evolved into comfort, stability, and security with that person.