The different keys do relate to emotions imo, although i think you have to be quite finely tuned to hear it, and it's more like a "mood" than an emotion as it is more of a guage. It seems to me that C major is neutral, and for every sharp added to the key signature it seems to lighten up the feel of it a bit (so G is a bit lighter than C, and D is a bit brighter than G, etc) and similarly, for every flat in the signature the feeling dulls a bit. I'd decribe it as getting happier and sadder, but that seems too extreme and slightly misleading, it's a bit different to that. It's also only noticable in changes from one key to the next, ie the music has to go from, say, C major to G major for G major to sound brighter, and it is extremely prone to ear fatigue. I only notice it the first few times i hear it until i've had a good rest.
I think it's due to the way concert pitch is tuned, or not tuned more precisely. It is a generic tuning that caters for all modes of all key signatures, and isn't actually in tune. To be absolutely in tune, you have to tune the instrument to precisely the key you are playing in (it's called tempered tuning for those who didn't already know this
) and if you do this for all keys in turn and compare them, they all sound equal (there's an option to do this on the Korg M1
, although it exaggerates and just sounds out of tune. i'm not sure about other synth models, but it's best tried properly on a harpsichord - they have real tempered tuning). I think that it is this generic concert pitch tuning that causes the slight change in "mood" as you add flats and sharps to the key, the further away from C major you get the more the mood of the key gets affected, because the nearer or further to/from the keys true tuning you get. It is more noticable when playing full chords in the higher octave ranges.
Don't expect to go and try this now and be able to hear what i mean, i have near perfect pitch and i think it is this sensitivity to pitch that enables me to distinguish it, although i could be wrong, it's just that i've noticed slightly out of tune parts in professional tunes before that no one else seems to have a problem with. As such, if you did write your tunes to take advantage of the different moods of the keys, most people would probably not be able to tell the difference anyway.
Anyway, i hope i've helped, but it's hard to explain things that you just can't really put into words easily.