system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Why is it that scientists make claims that planets are uninhabitable because there are gasses, temperatures, etc. that wouldn't support human life? This surely shouldn't be one shoe that fits all.

Surely, all creatures (alien or human) require different 'things' (water, oxygen, etc for humans) to survive. But, those 'things' might not be essential for alien life so why is it that scientists say that certain planets aren't fit for life because they are inhospitable for humans.
gpt
there are estimates that you need a certain variety of molecules to form what we would consider as life. i. e. on earth, carbon bonds to different stuff, and forms things like DNA, proteins, and so on. but we can't make a, say, sodium-based lifeforms because sodium doesn't bond to necessary amount of things, and those to which it does bond don't posess necessary properties. 

we could suggest that maybe our estimates are wrong and life is still possible with a non-carbon set of molecules - but it may be so different that we wouldn't be able to simply recognize it. 

so, we are in search of life that we can recognize, and that is carbon-based life. and carbon-based life requires certain conditions and leads to certain observable consequences in the planet's atmosphere etc.