Why does love fade?

I saw this couple who dated for 10 years until they got married. Obviously their relationship was still passionate and strong by their 10th year. Then 2 years after the wedding , the husband cheated. With a new born baby , the wife tried to make it work. They had their daughter a year later. But the husband simply lost his interest in spending time with her. Another 3 years later , they had no other choice but to divorce. According to her , he didn’t even love her too much and that she didn’t leave him cus she’s afraid of being alone.

Is passion and love supposed to fade? It must take intense passion to sustain a relationship for 10 years. but how does it move from intense passion to “I don’t love you that much anymore” in just a few years after marriage? What has changed?
Why does love fade?
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