why should I care

I know he is in another state and not in the one he told me he was in, he is with another women he met on line.  I have known since last week and have gone from indifferent to, so be it, to I hope she gets to see the real him. And in between the anger that he is a real bastard.  I am now just left with the anger.  I don't know if it bothers me that he is with another women once again, or if it bothers be of the way he just doesn't care who it effects. that he just is so indifferent to what he has left behind.  I am so angry at him, and I shouldn't be I should be at the point in my life that I shouldn't care.  I don't want him back I am grateful that he is no longer in my life controlling me and making me feel as if I am crazy so why do I care what he does why?