Why would the Lord expect me to be steadfast and immovable always abounding in good works?
I think can partially be answered in the scripture itself. Where it says, "...that Christ, the lord God Omnipotent, may seal you his, that you may be brought to heaven, that ye may have everlasting salvation and eternal life..." Christ wants us to be his. He wants to bring us to heaven so we can have everlasting salvation and eternal life. he even said that it is his "work and his glory to bring to pass the immortality and eternal life of man." Another scripture also answers this question very well. See Helaman 5:12 of the Book of Mormon.
Scripture: "And now, my sons, remember, remember that it is upon the rock of our Redeemer, who is Christ, the Son of god, that ye must build your foundation; that when the devil shall send forth his mighty winds, yea, his shafts in the whirlwind, yea, when all his hail and his mighty storm shall beat upon you, it shall have no power over you to drag you down to the gulf of misery and endless wo, because of the rock upon which ye are built, which is a sure foundation, a foundation whereon if men build they cannot fall."
If we are steadfast and immovable with Christ as our foundation, we will be able to withstand anything the devil throws at us and we will be happy because he will have no power over us for he "seeketh that all men might be miserable like unto himself" So, i think that the answer to the first question is simply that God wants us to be happy and that by staying steadfast in Him we will have everything we need to be truly happy.