There are lots of reasons that many people - especially young people - don't really believe in God. This posting lists a few of those reasons. Later posts will address several of them in considerably more detail (and offer some alternative reasons for belief).
1. We tell our kids all about Santa Claus, and the Easter Bunny, and the Tooth Fairy. Eventually, however, we confess that we weren't telling the truth. We tell them about God, too, and for some reason this time we expect them to believe us.
2. At various times in human history, people worshipped animals, the sun, the moon, etc. We worship a God that none of us has ever seen, but that's different (isn't it?).
3. Christians believe the Bible is the Word of God. Muslims believe God essentially dictated the Qu'ran to Muhammed. Mormons believe the Angel Moroni wrote the Book of Mormon. We tell our kids that the Muslims and Mormons are wrong but we're right.
4. We teach our children that God is all-good and all-powerful. But they also learn that bad things happen all the same.
5. Some indisputably-smart people have rejected the existence of God, sometimes implying that believers must not be very bright.
6. We train our young people to think critically, to keep an open mind, to challenge assumptions. But then they bring those skills to bear on the religious doctrines we want them to embrace.
7. Secular culture is not exactly supportive of religious faith. This needs no elaboration to anyone who watches even a little bit of television.
8. Purely-scriptural arguments often don’t make the grade -- many young people don't buy the notion that "God said it, I believe it, that settles it!"