So I've been playing the Mass Effect series and doing astronomy research for college and I realized something:
We assume for any kind of life to evolve on other planets in the universe they have to be like us. However, what stops them from evolving to live in a Nitrogen based environment or another gas based environment and also, what stops life from evolving on planets with stronger or weaker gravity or on planets with thicker atmosphere.
This assumption leads scientists to believe life isn't likely on other planets because of how we limit it.
In theory Life could evolve anywhere at any point if the conditions lined up.
I personally believe in aliens because of a quote,
"Either we are alone in the universe or we are not and both options are quite daunting and scary."
I refuse to believe that in the universe and even a galaxy as large as ours we are alone, its preposterous in its probability.