Though we cannot assume each language is as complex as the next, we can say confidently that complexity in a language is a natural process, as will explained below.

The complexity in a language is a testament to the maturity of a language. As languages get spoken, over periods of time they will begin to grow to be be more specific depending on the needs of their it’s users. Also, as the language grows in number of speakers and gets passed down the generations, more accents and sociolects are created and enforced or rejected that can often cause phonological and structural changes to a language. For example the Great Vowel Shift in English between the 12th and 16th Century in England.

Here is a picture explaining that vowel shift. This image shows the vowel shift in English which resulted in many sounds changing as you may note from the image below. This change did not occur all at once but took 4 centuries before it stabilised. Due to the large timeframe of this shift, it cannot simply be attributed to either political or social changes. Rather it was an amalgamation of causes that resulted in this shift. As such it is seen mostly as a natural shift that languages go through. In many cases, languages shiftt to what is easier and requires lesser effort on the part of the speakers to produce.

The borrowing of words from other languages especially in this increasingly connected world can also cause languages to become more complex. You can see the effect on the complication of language like in the Goose-Geese but Moose-Moose, singular and plural as explained in this succinct tumblr post.

If languages never matured and became more complex they would remain as creoles. Evidence that complexity is a marker of maturation are languages like Tok Pisin that start off at a rudimentary level  and are Pidgins but later gain complexity and become Creoles. More on this can be found in 5. Role of Pidgins and Creoles in Linguistic Complexity.