Why do fewer relationships continue (and progress) after the honeymoon period?

Fewer of those boyfriend and girlfriend relationships progressed and make it past the honeymoon period, into commitment, mature love and actually having kids with them. This is where you actually see each others' flaws and deal with the challenges. You're over the initial euphoric stage because that doesn't last forever.

Sadly many (more than half) relationships end after the honeymoon period, more than in the past. Why? Why do fewer survive past the honeymoon period?

Why do fewer relationships continue (and progress) after the honeymoon period?
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