Different media are able to show different things -- novels show interior thoughts, plays need to focus mainly on dialog, and movies/TV are largely driven by images.
Particularly, movies and TV have a huge challenge in communicating the interior thoughts of characters -- this is one of the first things you learn in screenwriting, when a particular story might be more appropriate as a novel instead.
But one of the tools which can communicate a character's state of mind is through music. At best, I think it's what you mean when you say complementary -- e.g. a character has to act confident but you need to know that inside they're nervous. But of course it can be a crutch for bad acting too -- if the actor is supposed to be communicating nervousness but isn't, so the music does it for them.
Ironically, animation seems to be a worse offender than regular movies. And sometimes the story itself simply isn't good to begin with, and does not give any decent justification for being emotionally invested in the main characters.
Not trying to pick a fight with you - I agree with your comment overall, but this is a culturally learned thing, very specific to Western movies and TV-series, and it is actually mildly infuriating to me. It is one thing to complement the emotions present in a scene, but being told what to feel is basically a cheap way to try to patch up bad storytelling.
Worse, it can totally backfire: for the dub of Castle in the Sky, the original composer had to fill in a ton of quiet scenes with music because (IIRC) Disney was worried about the audience feeling awkward about not being told what to feel. They also added background chatter, and worst of all: filled in scenes that were quiet on purpose with all of these things. While the orchestral score sounds great in and of itself, in most scenes all that added audio ruined the mood of the original.
However, I can't deny that what you state is what the audience expects, and that it "works", so sadly you're still right.