In short, it's not. 欢 and 喜 are synonymous adjectives that are juxtaposed to form a verb.
At the beginning, they were two synonyms that are often used together.
Then it becomes a habit. 欢喜 and 喜欢 were used interchangeably, with the meaning happy, joyful.
As a word, both 欢喜 and 喜欢 are first documented in Han Dynasty.
欢喜 is, however, more frequently used, especially in the translation of Buddhist scriptures.
- Mostly, 欢喜 retains the meaning of happy, but in some examples, standing alone, it's difficult to determine whether it's "happy" or "to like", such as
In （明）《拍案惊奇》, the meanings are both associated with 喜欢.
It's clear if an object is followed, since an adjective cannot have an object. It has to be a verb, meaning to like.
If there's no explict object, it's sometimes hard to decide:
From this, a logical development of the semantics of 喜欢 could be as follows: as a compound adjective 喜欢 first extends from happy to liking, then it changes part of speech to include the transitive verb to like. In many dialects and also the common tongue, 喜欢 has lost the meaning of happy, while 欢喜 only retains that meaning.
Notably in Wu Chinese, 喜欢 and 欢喜 still have the same meaning, with 喜欢 being formal and not usually used colloquially.