Have you ever got mad or irritated with your friend just because he's late, he did not keep his promise, he didn't fufil a task or had drag it...??? I guess you'll be really pissed and annoyed by it... He or She might be unreliable..

They might partially be wrong sometimes.. But sometimes do we give a chance for them to explain

The following might sound like excuses but have you considered any of them:
I fell sick or feeling pain --- I might be really sick like a fever or a bad flu or chest and muscle pain and i'm not able to complete in time as i had to rest at home...It was never easy to work in such a condition... youi have to live through it then you know.. i have...

I have to go to a concert cannot attend to you --- I bought it beforehand before everything was made, i cannot possibly give away my 100 dollars tickets.. i need to rest after aaving to go thru so much can't i take this break? It would then be better

I have to go visit my relatives--- Family always come fiirst, it migt be someone's special day.. Family is what u need close to your hearts

there are lots more...
They migt be excuses but try considering them, don't always jump into colclusions that it's an excuse and think they do not want to do it.. Often ppl wish how much they can do.. But have other things that are often prepared beforehand and are important.. Yes stuff made beforehand can wait and postpone but it's a promise done before everything is decided just try to make time for it to complement...