Joe,

Part of it is "what you are used to." If you listen to music from other cultures, the "harmonies" (i.e. the chords) they use vary significantly from Anglo-Saxon ones to which we are accustomed.

Any group of notes can be a chord. My ex-wife's uncle is a musical prodigy (he formerly taught organ at some university). One of his parlor tricks was to ask three people their favorite note. He would then improvise a tune on the piano based on those three notes. It really did not matter to him what the three notes were as long as they were from the intervals on the scale.

Move into jazz and many chords sound dissonant to the ear, but their dissonance is used to make music.

Roger in Baltimore (and headed for the Getaway)