The 双喜 (shuāngxǐ) symbol is arguably the most recognizable symbol of Chinese weddings. Its importance is deeply rooted in the cultural emphasis on family, harmony, and auspiciousness. According to a popular legend, the symbol originated with a young student in the Tang Dynasty. On his way to the capital for the imperial examination, he fell ill and was cared for by a herbalist and his daughter. When he recovered, the girl, who was also a scholar, challenged him with the first half of a couplet. He couldn't solve it immediately but promised to return. At the examination, the emperor himself challenged the student with the first half of another couplet. The student realized the emperor's line was the perfect answer to the girl's riddle, and the girl's line was the perfect answer to the emperor's. He used each to answer the other, impressing both the emperor and the girl. He won the top position in the exam and the girl's hand in marriage. On their wedding day, he joyfully wrote two “喜” (xǐ) characters together to celebrate his dual good fortune.
While it's a famous term, 双喜 (shuāngxǐ) is not typically used as a spoken word to describe one's feelings. You would not say “I feel very 双喜 today.” Its usage is overwhelmingly visual and symbolic.