This past May, my doctor told me that I have high cholesterol. I was given a prescription and was told to change my diet and to exercise at least 30 minutes a day. I have since been taking the medicine faithfully, and have modified my diet a lot. I still eat some things that aren't the best for me and don't always think about portion control. And I haven't been exercising much. Possibly because I don't believe it will help, or more accurately, that I just don't want to.
Liken my situation to a hypothetical individual who has just discovered the gospel of Jesus Christ from a missionary. This missionary tells the individual that he has a condition that, if left untreated, may cause spiritual death. But that it can be treated by praying, attending church meetings and studying the scriptures. If the person prays and attends church meetings, but doesn't study the scriptures, can they truly be saved from the worst possible outcome? Yes, probably. But could they change the outcome completely if they follow ALL of the advice given? Definately.
I went back to have my cholesterol checked and found that yes, it had decreased significantly, but was told that it's still higher than it should be and that I'm still in danger. I cannot tell my doctor that because I've changed my diet and taken my prescriptions alone that I should be spared. I've not fully followed all of his instructions and I can have no promise from him that I will be healthy, until I do that which he has required.
I wouldn't want to get to heaven and find out that I made it most the way there, but because of my lack of desire to follow all the gospel principles, I could not be completely saved and that I would not be able to qualify for the highest kingdom in heaven. It would be sad to report to the Head Doctor (Heavenly Father) and tell Him that you didn't follow His instructions, nor the instructions given by the Department Head (Jesus Christ), nor the resident physicians (prophets, missionaries, etc.) But yet you still believe you should qualify for the best outcome.
Hopefully I will change my desire to exercise...and to completely follow all of the gospel principles, so that I can continue to live life here on Earth...and will qualify for a nice mansion to retire to afterwards.