likely cause the chances of a life supporting planet having the same gravity, oxygen ratios...etc on top of them not being any lower on the food chain or any other environmental dangers preventing them from developing and evolving...are pretty slim really.
Consider that there are over 100 billion galaxies, each containing billions of stars. Now certainly each star will not have planets, let alone one in the goldilocks zone, but to me the numbers alone suggest there are many planets the right distance from a star to support water and life. Mathematically speaking, the chances of life elsewhere in the universe is extremely high. Now how evolved that life is, is another question entirely but don't forget how old the universe is, and how relatively young the earth is. Even if another intelligent lifeform had a measly 500000 year head start, the implications are astounding.
20-30 years ago you would be hard pressed to find a main stream scientist to admit the likelihood for life elsewhere as being great, now you would be hard pressed to find the opposite.