Regardless of the "god" portion of the question, it's really a simple answer: Because humans often choose suffering for themselves, sometimes even attempting to spread it to others.
Pain in life is inevitable but suffering is not. Pain is what the world does to you, suffering is what you do to yourself. Pain is inevitable, suffering is optional.
~ Buddha
People suffer because they are caught in their views. As soon as we release those views, we are free and we don’t suffer anymore.
~ Thich Nhat Hanh
So true! Or.. is it their upbringing and the world in which they liventhat made them so?
Definitely, it's their parents, school, media that teach them to choose suffering... But in today's world where every text and teaching is available almost instantly for free - ignorance is always a choice.