I can try to explain why. So far only known life form is our carbon based, situated exclusively on planet Earth - it includes every living organism on Earth, from amoeba to whale. We do not know it for sure, but it is possible (but unlikely) that it is the only life form existing in the universe. More likely is that life is like a sparkle in time in space - that there were (insert arbitrary large number) of life forms before us, and will be even more after us, but on cosmic time scale life expectancy of average life form is very small, so most of time/space is kinda lifeless and boring, and, most important of all, lacks an observer.
It's easy to think about observer as about god - it's something that everybody have or it lives inside everybody who's alive, it does not include any part of personality, it does not have any properties at all - so everybody have the same observer, or, you can say, it's one for everybody. If you think about it this way - it does not die after your death, so it makes you almost immortal - if you define 'me' such it does not include your personality, which I believe is right - as personality is just a sum of your genes and previous life experience.
So, what can be our purpose in life, and what can be the purpose of a humane race? Based on above I believe it should be protecting and expanding areal of carbon based life form. This runaway greenhouse scenario will end up with earth without liquid water on a surface - it can possible kill all life as we know it, so we should try to prevent it, even if it will happen after our personal deaths. Anyway, on a long enough time line, chances of survival of life on planet Earth drops to zero - so I believe we should do what we can to extend life, not necessary humans - maybe just a seeds from which evolution can begin - to as many planets as we can.