God never said only those who have never had a doubt will get to heaven. If you have ever had a doubt consider yourself in good company (Moses, Peter, John the Baptist, Elijah, and many more). The question is not if you ever have a doubt, the question is what you do with them when you have them.
Doubts arise from many things but two of the main causes are fear and lack of information. I would contend that the first comes from the second. So at the end of the day, doubts spring up from a lack of information or not having all the answers. What is interesting to me is that God made us that way. He purposely doesn’t give us all the answers and that is where faith steps in. Don’t you think if God made us not to know everything that he would expect us to have problems with the consequences of living that way?
I think doubts are far more common, normal and natural than we often think. There are at least two things that come together that hinder us from having a healthy way to deal with doubt. 1) We don’t normally have people express their doubts out loud because of the uncomfortability we have associated with it. 2) We typically avoid the stories in scripture were people had doubts. The result is we have a lack of knowledge of how to deal with doubts when they arise in a way that is healthy and scriptural. I think this starts early. We ought not to avoid these stories in teaching our children both at home and in Bible class and we also need to model for them, as adults, how to deal with our doubts and questions when appropriate and in a way that builds their faith rather than clouds things up.