My wife returns this evening from two-weeks away. Which, in the way that such things do, makes me wonder why ‘u’ is the least cheerful vowel in English.
As most people who learnt any chemistry or physics will recall, the simplest atom is hydrogen; chemical symbol H. So, we will take hydrogen compounds as our simple test.
To balance statistical validity and speed, we will form each compound three times.
HA HA HA = laughter
HE HE HE = laughter
HI HI HI = a greeting
HO HO HO = a very merry laughter
HU HU HU = a breathless owl or a determined investigator
So, most vowels combine to form laughter; and it would be very rare to use ‘hi’ to greet someone you weren’t comfortable with, so that seems more happy than not.
U however forms a compound of more at home in thrillers, mysteries, and ghost stories; potentially enjoyable, but not cheerful.