Every time you promise something and don’t deliver on it, it makes it harder for us to believe you’ll deliver next time.
“I’ll deliver those leaflets for you”, “I’ll score those arrangements for you”, “I’ll take 10 tickets from you for the gig”.
Your word is not just your bond, it’s you.
Thankfully however the inverse is also true – if you do deliver, it makes it easier for us to believe you’ll deliver next time.
If you say you’ll sort something for someone, for your and the other person’s sake, just sort it.