It's a hard to explain phenomenon.
Whenever you start a relationship, both the parties will try their hardest to appear in the best light. They will overthink every choice and every movement.. "oh no, did i say x and x wrong" or "did they think what i did was weird".
But after that initial point, when you both stop over thinking about these things, you begin to appreciate whenever your partner shows any imperfections or weirdness.. It feels "natural" or "normal"?
It feels like you finally start seeing their real colors - but that sentence is for the first time said in a good way.
It's like falling in love all over again, but with the same person. You learn about cute quirks or imperfections in their behavior and looks.. That one crooked tooth, the one mole on their belly, the way they laugh or smile. But you don't think of it as imperfections, but as something that as a whole makes them perfect, and something that you love about them.
You go past trying to be ideal, and instead, you be yourself. And it's up to your partner whether they accept you for who you are, or if they only loved you for the perfect image that you put up at the start.