A and B work in an office together. Tea, coffee, milk, etc are brought in by members of the team on an ad hoc basis and shared.
A brought in UHT milk this morning. B doesn’t like UHT - A knows this but always has UHT at home so it’s convenient. At lunchtime, B asked A to pick up some normal milk from the local supermarket while she was out.
A declined because she can’t be arsed going slightly out of her way and queuing in a supermarket when she already brought some milk in this morning.
Yabu - A is a meany
Yanbu - B can get their own milk if they don’t like it.