He – Why didn’t you call?
She – because you said you’ll call me.
He – It skipped my mind, you should have called me up.
She – well, I don’t think that’s a good idea.
He – why not?
She – I know I am disposable for you. You like her, the minute you tell her and you both start dating you’ll not even care about me. I am your backup for when you have no one else to hang out with. I am your backup when you are bored and there is nothing else to do. Don’t try to deny this; we both know I am right.
After that, they both knew the truth and just chose to ignore it till the time the inevitable happens. All this because he wanted someone to hang out with when bored and she just wanted to spend as much with him as possible.