Why my mom did my dad like this why?

my mother cheated on my dad and tried to hide it my dad is good to my mother very good to her but me and my brothers and sisters cant understand why she did him like that dad is divorcing our mother now we don't even respect her. she has no friends at all now why would she ruin her life as well as ours thanks a lot what makes her that sorry she is in her . 40.s late forties the guy she cheated with was our big brothers friend he is .18 years old