Love disappears after marriage, why?

The number of relationships that has hit the rock in the 21st century is almost uncountable, it has left many people broken, some after their failed marriage commit suicide, some are not and will never be the same again. The question I keep asking myself is why does the love just disappear after two love birds get married?

I will give 3 scenarios that leads to most failed marriages.

Scenario 1: In the civilized world where a man (Frank) work all his life, gets married to the love of his life (Mabel) and within a year or two, Mabel divorces Frank for a reason or two, Frank loses more than half or all he has worked for all his life. Frank is ruined, for life.

Scenario 2: Johnson and Cherry are madly in love, they believe they are meant for each other and they get married within a short period thinking they are ripe for marriage. 3 months into the marriage they notice the love and excitement is not there again and they start regretting ever getting married to one another, divorce is the next thing that comes next.

Scenario 3: Sandra and Lamar are in love but Lamar does not know Sandra has a bad habit, Sandra tells Lamar about the flaw/bad habit she is battling with, Lamar is madly in love, he thinks its a small issue and he turns a blind eye to it. Months into the marriage Lamar is tired of that bad habit he turned a blind eye to and he lose(s) interest in Sandra and the relationship as a whole. Lamar files divorce and the relationship ends.

These 3 scenarios are what I could think of right now, add yours if you know more, let people learn!!!
Thanks for reading.