can anyone explain to me why music in a minor key tends to be depressing? i read in my health book that we analyze the “mood” of music before we even hear the words… kind of interesting to me at least.
Realize first off that “major” and “minor” are terms that refer to a specific cross-section of western art music. These terms are unknown in Middle-Eastern or Asian music.
What you’re left with is self perpetuating conditioning.
I agree wholeheartedly that we parse the “mood” of music before lyrics, but we parse through the filters we acquire through immersion in a culture and its art.
I.E. - Music in minor keys tends to be depressing because we think that music in minor keys tends to be depressing. There is nothing inherent in the minor mode that causes us to be depressed.
That was a little terse - I wanted to get the first word in!!
“Specific cross section” is a little limiting: major and minor modality obviously has greater influence than just from “Western Art Music” (AKA "Classical) - a rock tune in a minor mode is much more likely to be angry/dark/moody than on in a major mode.
But the point that it is a cultural phenomenon stays: Middle Eastern music does not use major/minor modes, nor does Japanese music, African, American Indian, etc: if you listen to that music and hear any angst it may be in opposition to the actual content of the song, yet since your ear is “tuned” to hear a minor third as “depressing”, you’ll hear it as depressing. This tends to indicate that the quality we ascribe to a mode is based more on the context in which we’re used to hearing it, than anything inherent in the mode itself.
I hope that’s a little clearer, and less abrupt! 
It’s probably self-perpetuating to some extent: composers know that minor keys are supposed to be “sad,” so they are inclined to write slower, “sadder” music in those keys. But it’s not necessarily the case that minor = sad and major = happy. Consider for example “Roundabout” by Yes; a very ebullient song set in E minor.
(Not D minor, which as we know is the saddest of all the keys… )
There are many things that make up music other than tonality (major, minor). Many things can make a song sound sad including tempo, rhythm, timbre (instrument), key changes etc. Tonality is just one of the tools that can help a composer achieve the sound that he wants. Of course there is plenty of music in minor keys which sounds “happy”, and a major key can be made to sound “sad”. A minor tonality is also good at being “sexy”. Much depends on the other aspects of the music and also on perception of the listener.
Maybe a better rephrase of the question: What makes a minor tonality sound sad?
I think it has much to do with the consonance and dissonance of various intervals. Dissonance can be described as the “clash” between two or more notes. Certain musical intervals have a “soothing” sound, such as a major third, because the result of the two notes played together dont create much dissonance at all. On the other hand an interval of an augmented fourth sounds very dissonant and can have an unpleasant sound.(sometimes refered to as the devils interval) The major and minor modes are constructed of these many degrees of dissonance giving each of them a distinct sound by emphasizing a tonic note (root note). With the 12 tones in western music, many different effects are possible when multiple notes are combined.
You might want to do a bit of research to get a better scientific and mathematic understanding of dissonance and how we hear it; I just dont think i could provide a good understanding of that here.
And of course as said above, music is subject to culture and all the music that you have ever heard before will influence how you will hear anything that comes after.