I thought my grammar was excellent BUT now not so sure:(

why is and necessary in some sentences and not others?

Boys, girls, dogs, and cats ....

vs

Bright, friendly, compassionate ....

2 answers

The first is a series of nouns.

The second is a set of compound adjectives.
I agree with you that your grammar is excellent.
when using series a Comma is used after each word regardless if it is a noun or adjective or a verb. Before the last word there should be the word "and".
therefore, the sentence should be as follows

Bright, friendly, and
compassionate.