@Habitat
Not quite - you can still have sad lyrics and use minor chords but have a happy melody
Without delving too far in to music theory it is the "feel" of the track that gives it a happy or sad vibe, and ths is usually due to the sum of all of the parts rather than a few individual things
TINA is actually spot on with her ideas - get in a happy frame of mind and write
The ideas in this thread should steer you in the direction you want to go though, keep things up tempo, lots of major chords, and keep the melody "light" and bouncy with lots of high notes
Bruno Mars has some great "happy songs" on his album - "Marry You" uses many of the devices that has been referred to
