The simple, sing-along, nursery rhymes and folk songs we learn as children, the cheerful, toe-tapping pop and rock we dance to, the uplifting sounds of a symphony: most music in a major key has a bright sound that people often describe as cheerful, inspiring, exciting, or just plain fun.
Music in a particular key tends to use only some of the many possible notes available; these notes are listed in the scale associated with that key. The notes that a major key uses tend to build "bright"-sounding major chords. They also give a strong feeling of having a Tonal Center, a note or chord that feels like "home" in that key. The "bright"-sounding major chords and the strong feeling of tonality are what give major keys their pleasant moods.
Listen to these excerpts. Three are in a major key and two in a minor key. Can you tell which is which simply by listening?