Why is language arbitrary?

Languages are said to be arbitrary because there is no necessary or natural relationship between the words of a given language and the concepts that they represent. For example, there is nothing in the word "tree" that connects it to the concept of a tree; which is why Spanish can use a totally different sign for the same concept: "árbol"; and so on with other languages.

Also, languages are arbitrary because the rules for the combination of signs in order to produce complete thoughts are different from one language to the other, and no set of rules can claim to be the "right" one. For example, in English you say "I like beer", whereas in Spanish you would say "Me gusta la cerveza". The translation/transliteration of the latter would be something like: "Beer is agreeable to me", or [is agreeable to me the beer], which sounds strange in English. And neither of these formulations has a better claim to accuracy, correctness or truth than the other.