what is mindset of people who breaks appointment without notice?

i work in small healthcare clinic we see about 10-15 people per day. when people make appointment and just doesn't show up, it's big revenue loss and also it means other patient couldn't have come since patient (who broke appointment) was taking that time slot. We call 1-2 days ahead to confirm. we understand things happens. But I'm not talking about people who calls us to cancel or reschedule. I'm talking about people who, (sometimes, even after he/she confirmed appointment 1-2 days ago), just doesn't show up and when front staff try to contact, no answer, no reply, as if the person just vanished. 

I'm not trying to get solution. I am just so boggled by this and want to know what is their thought process. Plz don't get offended. for those of people who breaks appointment often, what is the reason? are you normally like that with all appointments with your friends? like, you set an lunch time with your friend at a restaurant next wk but just don't show up without any notice if somethings comes up or you don't feel like going? 

what is your thought process? is it that

1) if something happens or if you just don't feel like going to the clinic, your mind just ends there and the thought that (clinic may lose money or there may be other patient who may want to come on that time ) never even occur to you?

2)or those thoughts occur to you but you simply don't care and see no need to let clinic know that you cannot come?  

3)or other?

I will thank you if you give honest answer