When it came to family, Wei Wuxian had a huge void in his heart. This stemmed from his upbringing; he lost his parents at a young age and then had to live under someone else's roof.

Jiang Fengmian and Jiang Yanli were good to him, and he would repay that kindness tenfold. Yu Zi'yuan was not, but he didn't hold it against her.

But the Jiang family could not fill the void in his heart.

He always kept the memory of his parents deep inside, never showing it to anyone.

And Lan Wangji, he made Wei Wuxian think of his own parents. This was Wei Wuxian's most deeply buried longing and most beautiful memory of what 'home' should be.

In a true sense, Lan Wangji gave Wei Wuxian a home, filling the enormous void of family in his heart.

Lan Wangji was not only Wei Wuxian's love, but also his family and his friendship.

Lan Wangji also had a void in terms of family, but his was only the absence of his parents.

As for other things, he had an older brother and an uncle, so he wasn't lacking.

Therefore, Lan Wangji was not as starved for love as Wei Wuxian, nor did he long for a home as much. Because, he had a home, he had family.

That was why Lan Wangji could give Wei Wuxian a home. He could give Wei Wuxian a strong sense of reliance, causing Wei Wuxian not only to fall in love with him, but also to let down all his guards.

Being with Lan Wangji made Wei Wuxian think of his parents. It made Wei Wuxian place him on the same level as his own parents.

At the same time, Wei Wuxian also filled the familial void left by the absence of parents in Lan Wangji's life.

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That’s why his love for lwj is so much higher than anyone else. Wwx found true belonging the way he found with his parents. 

The above expert is from chinese social media and it’s very common there regarding this 

Why Lan Wangji holds highest place in WWX’s heart