The Surprising Ways Mars is Hostile to Life

I don't get why there's this assumption that other planets are unsuitable for life. 

1. Life on earth developed and evolved through earth's unique and changing climate over eons. In other words life on earth is  designed for earth.  It's akin to asking if humans can  live at the bottom of the Mariana trench.

2. That said,  life unique to other planets can exist. It may be strikingly different to life as we know it or not as varied as life on earth. May just be microbes or just simply unrecognizable to us. 

3. Despite our perception of other planets being inhabitable, the are many examples of  life forms on earth that not just survive but thrive in regions on earth once considered impossible for life. E.g near active volcanoes or sulphuric vents at the bottom of the sea.
So to assert a planet cannot support life because we deem the climate extreme is illogical.