What are the reasons behind monogamy in humans? What makes human being in anger or in mental pain once his/her partner leaves? If you think logically we can replace our partner with a new one or if alone also doesn't make any difference. 

From another perspective a person having multiple partner increases the chance of having better and more offspring, so by evolution this should be a happy emotion than a sad one. 

So my question is why is infidelity bad and not a good thing.
First off, I think monogamy is a socially and culturally determined construct in man. 

In the animal world both monogamy and polygamy are encountered. Monogamy increases the chance of the survival of individual offspring, as dad can help raising the offspring  and can defend the family. This increases fitness.

On the on the other hand, polygamy increases the spread of the (male) genes and hence increases the chance that the male gene pool is spread. 

The relative benefits of both strategies depends on many factors, including resource availability, offspring need, parental care, and mating dynamics (Klug, 2018). 

Reference
- Klug, Front Ecol Evol (2018); 2018.00030