There seem to be three reasons why people believe in a god. Indoctrination, ignorance of facts and/or wanting to believe.
This topic seems to have brought out the last two in people.
People want to have a purpose, they want to have a meaning to life, so they want to believe in a god. Most of the time, they want to believe in the Biblical god (due to indoctrination), which means their purpose is to give praise to a being who has killed billions, in the hope that they will avoid his wrath upon dying. This is the meaning of their lives.
My personal 'purpose' is to be happy and help others. The religious can claim that is their purpose too, but they conveniently have to ignore or try to justify certain rules in the Bible which go against that. Others can be moral for the simple reason that it's beneficial to us all to be moral- not in order to reach a celestial realm where God is to be worshipped for eternity.
I believe making our own meanings is better than having our choice stifled by a first century book. If we choose the first option, our personal purpose is limited only by our imagination (and possibly the laws of physics). If we choose the second, our purpose is to see the world through the lens of a book that talks about a talking donkey, a man living in the belly of a whale and magical hair that gives strength as real historical events. The desire for a life after this one seems to be so strong in some that they will accept these things actually happened (simply because they are written in a book), yet ignore all the evidence for the Big Bang and evolution, which can be seen in museums throughout the developed world.
It may be a little scary to think of ourselves as incredibly lucky to be here. An asteroid could wipe us all out any day, a disease could kill us all. Our own death may be scary, or we may think that the time we have isn't long enough. I prefer to see life as precious because there was more chance of us not being here to experience it. Every sperm that doesn't make it to an egg is a life that will never happen, every miscarriage is a person who wasn't lucky enough to make it. We should not be bitching about the shortness of life we have. For every one of us that is born, billions, trillions, probably more, are not born. They experience nothing.
To want more after this life, and to feel short changed if such an afterlife seems improbable, is really pushing our selfish tendencies to new heights. Not only that, but people wouldn't even be satisfied with an extra ten years, fifty years, 500 years even. They want to live forever, and may even take offense it people try to take away their selfish hope of an eternal life.
If we accept the standard Christian teaching, we live in a universe in which God has set things up so that we suffer from time to time and where most of the humans on Earth will burn forever for worshipping the wrong god, or the right god but in the wrong way, even though there is no more reason to believe in one over the other. This is supposed to be comforting?
On the topic of being ignorant of facts, many people think things such as evolution couldn't have happened because they have the wrong idea about evolution. The debate has been settled, and was settled long ago. Evolution is a fact. People don't accept it is a fact because it goes against their book, or they accept the distorted view of what evolution is from creationists with an agenda (we didn't evolve from monkeys!), or they don't know how to buy a ticket to a museum where countless fossils are on display showing how species have changed over time.
The proof exists, but due to indoctrination and wanting to believe in a god, such proof is ignored. We do not need a god to have a purpose, we should be happy that we get the amount of time to be alive that we do get (most animals live much shorter than us) and we shouldn't deny ourselves reality because it may be hard to face at times.