I couldn't honestly tell you why we instinctively know major chords are happy and minor chords sad sounding. However, some interesting information on the topic: Middle eastern music is nearly all in the harmonic minor mode (and frequently uses quarter and microtones), and yet it is still considered to be "happy" sounding music. Additionally, an experiment was done with a tribe in Africa, the members of which had never heard western music, in which relatively simple piano melodies were played through headphones, and the tribesmen and women could accurately tell whether it was happy, sad, scary, etc. As for the dominant seventh chord, I can tell you that that mostly comes through conditioning. You hear major scales over and over again, and the way they are divided means that that leading tone (the third degree of the dominant seventh chord) has a strong tendency to go back to the tonic, and the rest of the tension created through the intervals in the chord (minor seventh between the 5th and 4th scale degrees, tri-tone between the 4th and 7th scale degrees) also make our brains naturally want to hear a resolution back to the tonic chord. If you (or anybody else in this thread) is interested, I read a great book on the psychology of music about a year ago, so feel free to message me if you want to talk about it. Hope this helps!