"So Happy!"', a phrase initiated by the legendary NintendoCapriSun, has many viable connotations and meanings, primarily used in reference to Yoshi. The first "So Happy!" was used in Super Mario Sunshine '07 Part 2: The "Fucking Yoshi" Incident.
Additionally, one is able to buy NintendoCapriSun shirts at his website, linked to his Youtube page, containing phrases such as So Happy! Along with this, NintendoCapriSun found that Yoshi's noise he makes when he jumps in Super Mario Sunshine makes Yoshi sound "So Happy!" and used in Banjo Kazooie when finding a jinjo, and used in Yoshi's Story when finding a super happy heart.
His playthrough of Yoshi's Woolly World also included a So Happy counter.
In his playthrough of Pokémon Sapphire he named his Ralts after this phrase.
In his playthrough of Yoshi's Story, he calls the Super Happy Tree the So Happy Tree and being super happy being "so happy". He uses the phrase a lot during the LP. The first episode is titled "so freakin' happy".