One of the biggest reasons people give up is unrealistic expectations.
And there are all kinds of these. Some of them are even reasonable. For instance, my wife has a number of quite understandable expectations of me that for various mysterious reasons may never be met. At least not this side of heaven!
All joking aside, though, unrealistic expectations can be an absolute killer. When I met with people coming into our church, I always made a point of saying they probably had some expectations that would never be fulfilled, if only because no church is perfect.