You're exactly 100% wrong with that assumption. If you accept the scientific evidence of a natural beginning of life (and the evolution of life) on earth, that evidence clearly indicates the probability of life on any other planet is ridiculously small.
That's ONLY if you assume that a planet has to be identical in chemical make up to Earth, and in the same proximity to a similar star. Even the tilt and orbit would have to be the same to produce the same lifeforms we have on this planet (which are uniquely adapted to this planet).
Even so, given the immeasurable numbers of galaxies and stars out there, it stands to reason that there are many planets that are a close match to Earth.
That aside, life has a curious knack for filling the oddest niches. A planet could have a different chemical make up, and be in a different proximity to its star, and still support life. It wouldn't resemble the life we see here, but it would be life.