The closer someone is to you, the more you will feel disappointed by their inconsiderate behaviour. Your expectations are different of them compared to others. You expect more. And you are disappointed when they fail to meet those expectations.
It's not even in a 'quid pro quo' way, where you expect that the way you treat people that they should treat you and the better you do, the better they should. No, you reckon if they care they should try harder and that their lack of consideration is a measure of what they think of you. You are not worth turning up for, not worth being on time for, not worth committing to.
Of course argument against this view will come from the closest relationships we have, where we end up taking each other for granted and 'don't sweat the small stuff'. But when the balance of the 'small stuff' either becomes so prevalent as to tip the scales, or the infrequency of meeting means the 'small stuff' is the 'only stuff', then it matters.
Blake (1908) was right when he said:
'It is easier to forgive an Enemy than to forgive a Friend.'
You expect more from someone who you have a relationship with and are disappointed more when they don't seem to care. It is that pedestal you put them on. Or the Wizard of Oz effect. You expect and are promised one thing and behind the curtain you find instead, over and over again, disappointment. The naked truth is revealed every time in the beam of the spotlight. They don't care about disappointing you, because if they did they wouldn't. But you keep turning up and pulling back the curtains. It's tiring but one day you might be surprised.
Blake, W. (1908) Jerusalem, The Poetical Works, Oxford University Press