喜 shows a drum (the top part, 壴) over a mouth (口) — the image of music and a smiling, singing mouth: celebration. It has meant joy and happiness since ancient times.
喜 is joy itself — and gets its fullest life paired with 欢 (next) as 喜欢 (xǐhuan), 'to like'. It fills happy occasions: 喜事 (a happy event, esp. a wedding), 恭喜 (congratulations). The doubled character 囍 ('double happiness') is THE wedding symbol, pasted on doors and gifts to bless a marriage. Red 囍 everywhere means someone's getting married. 喜 is the character of celebration.
A drum over a singing mouth (口) — celebration, JOY.
囍 — two 喜 side by side, 'double happiness' — is THE Chinese wedding symbol, pasted in red on doors and gifts. See it and someone's getting married.