First, wickedness never was happiness. What an embarrassingly simple, straightforward truth. Yet so many do not know it. I think it's because the world's/Satan's message is so loud and complex that wickedness is happiness. The world says drugs, sex and rock and roll is fun and brings you happiness. It's portrayed in every form of media. People seem totally at ease, confident, happy, and fulfilled when following the ways of the world. We see it so often, many people become convinced that these things bring happiness. People try to treat sadness, pain, and emptiness with fun. They become jaded, confused and disillusioned when they feel worse instead of better. Because what they really need is to seek peace and healing by following the commandments and using the atonement and Love of the Savior. It may not seem as exciting, but it's true, and it's what brings happiness and contentment.
Second, each person is responsible to live according his/her own light and knowledge. Understanding this allows you to love other people. Perhaps people that are not living all the commandments, but are fully living according to their own light and knowledge, view people who are keeping the commandments a little more fully and think that they are being pretentious. If you followed commandments that you had no faith in, it would be a little pretentious. People who are strict followers look down on others and are annoyed that they don't sacrifice and step up to the plate and live a higher law. They think they are lazy and irresponsible. If you had a great knowledge, it would be lazy and irresponsible not to live what you know to be true. If people understood that not everyone has the same amount of light and knowledge at this moment, it would be easier to understand that it's extremely likely that they are doing (with their personal capacity) what they believe to be best, and who can blame anyone for that?
Third, keeping the commandments not only bring extra, unrelated blessings; they are blessings in and of themselves. Some people think that this life is a test, and once we get to heaven we can do whatever we want. Not so. This life is a time for us to realize that we love the Celestial law because it is good for us and for others. Keeping the commandments can sometimes seem unpleasant because it requires sacrifice and delayed gratification. But in the end, keeping the commandments brings us maximum happiness. If you kept the commandments 100%, you would be 100% happy.