You ask an excellent question, but one that's not easily answered.
To begin with, it is *not* fair to say that a certain type of meter (rising, falling, etc.) or a particular meter (trochaic octameter) always means something specific about the poem.
I'm sure that with a minimum of research, you could find counter-examples of what you proposed--that falling meter indicates sadness. Instead, it's essential to look at each specific situation and to try to observe how sound and meaning work together.
Here are a couple of examples:
Once upon a midnight dreary, while I pondered weak and weary...
Poe's "The Raven" has a clear tone of despair, which is actually the main theme of the piece. The trochaic meter here (ONCE u PON a MID night DREAR y) might be said to enhance the feeling of dread. Since the feet are short (two syllables each), they seem also to propel the narrative forward quickly toward an inevitable unhappy conclusion.
He clasps the crag with crooked hands;
Close to the sun in lonely lands,
Ring'd with the azure world, he stands.
Here we have mostly iambic tetrameter, which, combined with the word choice, gives this famous piece by Tennyson, a regal feeling. The iambs are interrupted by anapests, though.
Close TO the SUN in LONE ly LANDS
CLOSE to the SUN in LONE ly LANDS.
Hmmm.... I'll let you think about why the interruption. Also, can you tell where this short poems is headed in its second and last stanza? Can you tell who the main character is? (No fair looking for the title yet.)