''When obedience becomes our goal, it is no longer an irritation; instead of                                           stumbling block, it becomes a building block.''                                                                                                                                                                  ~James E. Faust

He is comparing a hard to get goal to an irritation. Usually, an irritation is some sort of discomfort. He compares an accopmplished goal to a building block. Building blocks are actually finished and complete. Once you accomplished obedience, there is no problem at all. This sort of irritation, as he says, fades away. Some people struggle just being obedient for at least once. It  takes time to achieve obedience, but you'll get there if you try. James is reffering the struggle as an irritation, and as you achieve your goal, all this irritation (struggle to be obedient) will go away. A stumbling block is a very negative adjective meaning difficulty to achieve. However once obedience touches you, you'll choose to be more respectful. Building blocks, are being referred to as relief. Relief and happiness comes once you overcome the struggle of being obedient. It is almost always important to be obedient or else the irritation kicks in, not literally. Still, it is very important to be obedient because we won't have a single struggle. Positive side affects may occur being obedient. Life is way more harder disobeying. It takes only few seconds to listen and obey. People act like it kills them. However, it is always better to stick with obedience because it won't feel like an irritation(struggle). It would feel like a building block(relief), if you choose the path to obedience. Therefore, choose obedience. 

   James E Faust
1920-2007