there's no short answer and i don't have much time but i'll give you a skeleton argument...
if there's no God, or at least some supernatural element outside of the physical universe, then you are left with a closed system, or an infinite system, or a loop. this system is the universe (i.e. all universes; everything natural). this system is governed either by determined cause and effect, or random chance, neither of which can produce truly self-conscious beings (humans. you.) your consciousness is therefore an illusion. you are nothing but a very complex machine, brought on by natural events. nature does not have a motive for what it does. it just does things according to law. the idea of 'purpose' is thus also an illusion, part of the complex machine. your brain invents purpose. what you do every day is for nothing.
now, if we're honest with our logic, the implications here are not particularly attractive to the human mind. thus we say things like "oh, i have purpose, just no overarching purpose!" that's b.s. you're only kidding yourself, and you'll figure that out soon enough if you stick with it too long. many believe that this very sort of logical honesty is what drove Nietzche insane.
so the options i have in front of me are these:
1.) there is supernatural stuff out there - we may not understand it, there may be a means of true purpose out there - human wisdom only goes so far.
2.) naturalism, choosing to ignore some logic so you don't end up insane.
3.) nihilism! *goes insane*