Obviously as a child I believed it. As an adult I really wanted to believe it and tried hard to. At that point I mostly wanted to get to paradise to be free from all the JW nonsense.
The parts that made it difficult to believe (or at least difficult to be excited about) was that the current life-supporting capability of the earth and indeed the universe would only be sustainable through constant divine intervention. It just seemed too inelegant for a supreme being to have set it up this way. Then there's the issue with the fact that essentially everything that I enjoy would be in direct conflict with the image of paradise that we're sold. Many things I enjoy pollute (either directly or indirectly) and couldn't be allowed, eating meat would be a thing of the past, etc. I hate menial work, so the 'build and have occupancy, plant and eat the fruitage' thing never seemed like the great scenario that people acted like it was. With the promise that we'd all be happy, I often wondered just how much of a lobotomy god was going to have to give me to achieve that.
To those wondering why you'd do it if you don't really believe - it's all I knew. Plus, it may have been the carrot, but even if you don't like carrots, you still have to worry about the stick.