Star Trek: The Next Generation introduces us to a wide array of alien life. Whether it’s returning species like the Vulcans and Klingons or one-time run-ins that either help or hinder Captain Picard and his crew, these life forms are relatively diverse, but also unconvincingly similar.
Obviously, the main reason for all the humanoid life forms is for the sake of plot flow and costume design. The Enterprise and its crew need to be able to communicate easily (with the help of their Universal Translator, which is basically Google Translate on steroids) with other life forms, new or familiar. We also need to be able to identify with these species on some level, and having human-like characteristics helps maintain that empathy.
But if life on Earth is any indication, the odds of running into bipedal organisms with virtually identical organs, speech mechanisms and appearances so similar to ours are exponentially low...http://scifiaddicts.com/star-trek-next-generations-aliens/