That occasionally happens to me. I come to expect it from those that do it often and just write it off as a flaw to their personality.
Why do people offer freely to give you something, freely promise to preform an errand then don't do it ? How do you deal with this?
The act in itself, is far more demanding then just simply offering the words of said action.
This is an impulsive reaction. Not fully weighing what the action requires, the need for "Praise" or good feelings (kicking those chemicals into the "Happy Place" in the brain) dwarfs the actual forethought of what it is that people "Promise" you.
"Not feeling well. I'll mow your lawn for you!" (Receives Praise and accolades):
I would have to get new friends. 😔
Or I just don't allow people to do for me. I am always too worried of putting someone out then I tend to decline offers of help from people. Unless it is something immediate like opening a door for me cuz my hands are full of groceries. (Plus I am a bit of a control freak! Lol!) But my advice is probably not the healthiest.
I would have to place it in context with what I know about the person. Does this person regularly promise and not deliver? Is this person a close friend? If this is just a one-time occurrence, I might ask and then let it go. If this happens on a regular basis, I would question my sanity.
It's hard to know exactly why people do this except maybe they want to be the savior of the situation but lack the fortitude to follow through. So it looks good on the outside but stinks on the inside.
It's one of the "small things" I try not to dwell too much on.
If the person has a history of fulfilling their promises, I'd just figure it was a fluke and go on as normal.
If they have a history of breaking their promises, I neither ask for nor rely on their promises. Nor do I spend any time telling them. They know.
Basically I don't ecpect anything from anyone, regardless what they say.