As has been said: Pray and read the bible
The short answer however, is: Sin.
Slightly longer:
If you read Genesis, you will see how many bad things (that we take for granted), that came into the world because of sin. If I remember right, even weeds were a result of it (!). In addition comes of course all things that are a direct result of the specific actions of humans. I too wonder how it can be like this, but I think the answer is: If God kept stopping us every time we are to do something evil we would be salves, or worse, we would have no control of our lives at all, and that is not what God wants, He wants us to choose to be with him and follow his will. If we were all able to do this all the time, there would not be those bad things.