Q. Why do you doubt the existence of a (a) Loving (b) All Knowing (c) Intelligent God or gods?
Loving - If the Creator is "loving", this word is different from the one found in the dictionary. If you put your child into preschool, and you see him start pounding on other kids, or if you see other kids start pounding on him, what would you do? Would you intervene? As a loving parent, yes you would. If you had the power to help your children avoid death, you'd do that too. If you had the power to help them understand the world around them and show them how to live the best lives possible, you'd spend quality time with them showing exactly what to do.
Would you kill your child for insulting you? Would you destroy your house and the entire family if they stopped listening to your wishes? Would you torture your kids for thinking the wrong things?
Almost certainly, no - you would not. Not if you loved them.
The Christian deity is not loving, regardless of whether he exists or not. {EDIT: the same holds true for almost every religious concept of God I've encountered. None are 'loving' in the way that we typically use the word}
All Knowing - this trait raises problems for the deity in question. Specifically, if He knows what He will do in the future, this means he can't be omnipotent; he can't do something different from what he has predicted he will do. Omniscience also raises some concerns for our ability to have free will, let alone be punished or applauded for the actions we take. If I am predestined to make certain choices, then there is nothing I can do to avoid that destiny. Why punish me for something I can not possibly change?
For me, omniscience is troublesome when it's combined with other traits. By itself, the idea is kinda silly, but otherwise not terribly objectionable.
Intelligent - this seems redundant (re. "all knowing"), so I'm going to assume you simply meant "exists". As in
why do I have a problem believing in the existence of a sentient Creator. Speaking only for me, I don't have a problem with it. My problems come from the kinds of things this Creator is attributed with. By itself, I consider it possible that we human beings were put here by an intelligent force. There doesn't seem to be much evidence for it, but hey, it's still a possibility.