Who's the judge?

Baby I'm yours,Be your forever, be your fling,Baby I will be your everything.
When you're in love, you're so blinded by everything, you tend to miss out their flaws and imperfections. But I guess if you really know the person well and love them, you would love everything about them and tolerate and understand their imperfections. 
I don't understand how someone can hate their ex so much when you've once loved them so dearly. I would say even when it's over with a guy I date, I don't hate them, never hate them. Things got together and fell apart for a reason. Both parties would be at fault. 
I'm just wondering when you date a guy, would you think, consider or mind his previous relationships? Judge how he treated the other girls he had. Would you? Would it matter? Or all that matters is here and now, and that it is you that he loves. Not knowing if a few months down the road, you would join the other girls and hate on him. 
Is it the same situation? Can it be compared that way? Or maybe their previous relationships never worked out for a solid reason and you shouldn't be worried about it. My thoughts would be, every relationship is different, even though it's the same guy. I guess it's cause I wouldn't want to be judged by my pass record. People change, hopefully for the better. 
Then again, you understand exactly how she feels, right then. Been there, not a very good time. It makes my heart sink just thinking about that feeling. When you question everything. Feel worst about yourself. 
In the end, it's whether you will learn to love them, or learn to hate them.