Part of parenting seems to be to tasting foods given to our children. We get that taste in before it is put in front of the child. Call it a toll or tithe.
My wife still does it to Galahad before letting him have an interesting new drink or dish. Or one that she likes but did not get.
They argue about whether or not she took too much. She calls it a bite or sip. He calls it a mouthful or guzzle.
He also complains about her taking the best candy from his Halloween exploits.
Primarily, he is suffering from loss aversion. He envisioned having it. Only now Mama is taking it away from him and diminishing the value.