One of the main reasons why, for instance, weight loss efforts fail to such a high degree is that dieters routinely start out with unrealistic expectations. They look at what's being presented to them by commercial weight loss programs or popular television shows and anticipate similarly spectacular outcomes in their own lives.
What is it that makes some athletes persevere while others give up? What drives an athlete at all? It's of course impossible to know if an athlete will 'make it' until they actually do but, in my mind, the root of this perseverance is planted in four simple things: a love of the sport, the desire to improve, being satisfied with small, incremental improvements and patience. In a word -- grit.