Why exes act like they don't care after break up?

after a break up why do ex's act like there perfectly happy about it specially after being in love with a person for a long period of time its been about two months with my situation and she acts like it don't even bother her but when we are in the same place she always looking at me if I'm walking toward her she is looking right at me and smiles what's up with that what do all you guys and girls think
Why exes act like they don't care after break up?
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