In chapter 1 of the book of Malachi, the Lord speaks through this prophet to the people of Israel saying that he has clearly loved them, but that they do not honor him due to their faults, the people were doing things as they wanted and not as God wanted. That is why the Lord reminds them of the love He has for them.
This entire chapter speaks to the people as well as a rebuke to the priests because the Lord speaks clearly to them, and verse 6 says:
A son honoureth his father, and a servant his master: if then I be a father, where is mine honour? and if I be a master, where is my fear? saith the Lord of hosts unto you, O priests, that despise my name. And ye say, Wherein have we despised thy name?
It was clear that the priests themselves were not making the sacrifice that the Lord deserved. That is why the Lord demanded a pure sacrifice before Him.