There are people who do not believe in the existence of God. And some people believe that, even if there is a god, he may not be the just, loving God that the Christians believe in.
To me, I firmly believe in God who is just and loving. Because, if there is no god and the world is just a matter of random coincidences, then nothing has meaning and this world is really not worth living in.
On the other hand, if there is a god and he is not just, then the good do not get rewarded and the bad do not get punished. Things are just too arbitrary and I simply cannot stand it.
Finally, if there is a god and he is not loving, then this world is just too cruel. Consequently I cannot expect to love and be loved. Then it is just too hopeless and I might just as well stop living now.