Sensory Disonance

A note entering an ear causes the ear drum to vibrate.  The in turn stimulate a patch of hairs in the cochlea in the inner year.  Different frequency will stimulate a different cochlea patch.  When several notes is heard, multiple cochlea patches are stimulated.  Patches for 2 semitone may have overlapped area.  The brain would need to work herder to identify them.  This biologically caused us to sense the sound combination of 2 semitone as dissonant.  

Interestingly, this same condition may appear to tones that are greater than semitones in the lower frequency range.  In other words, chords in lower frequency range did not sound as smooth as the same chord in a higher frequency range.  That's why musician does not play chord with bass instruments.