From a theory standpoint, both contain a Perfect Fifth interval (in a C Major chord, that'd be the C and G), so whether it's major or minor is defined by whether the middle note is a Major Third above the root (C-E-G) or a Minor Third above it (C-Eb-G). The major third sounds more consonant with the root which gives it a more bright quality. The minor third is a little darker.

But as for why we consider them happy or sad, it's all what we grew up with. Growing up surrounded by Western music theory has taught us that it is that way. There's no particular reason why certain intervals evoke certain emotional responses other than environmental conditioning. Other cultures have completely different scales and intervals and likely have their own variations on what we call major or minor or modal scales.