In Zenos' Allegory of the Olive Tree, the Lord asks His servant the question: "Who is it that has corrupted my vineyard?" The servant answers with another question: Is it not the loftiness of thy vineyard--have not the branches thereof overcome the roots which are good? And because the branches have overcome the roots thereof, behold they grew faster than the strength of the roots, taking strength unto themselves, behold, I say, is not this the cause that the trees of thy vineyard have become corrupted?" (Jacob 5:48 emphasis added)
Another reference to taking strength unto ourselves is found in Helaman 4:13 which says that when we boast of our own strength, we will be left to our own strength. By being left to our own strength we will not prosper and will become corrupted. This description is associated with not only the pride of their hearts, but also the denying of the spirit of prophecy and the spirit of revelation, descriptions of people who no longer believe in the power of God, having chosen to rely on their own strength and power. (Helaman 4:23)
Corrupted is also used in the scriptures to describe the people in the days of Noah before the flood, and specifically "...the wickedness of men had become great in the earth, and every man was lifted up in the imagination of the thoughts of his heart, being only evil continually" (Moses 8:22).
This particular type of wickedness is consistently described in the Book of Mormon and other scripture, and particularly describes the conditions of the last days, not only among the world but among members of the Church as well. "For they seek not the Lord to establish his righteousness, but every man walketh in his own way, and after the image of his own god, whose image is in the likeness of the world, and whose substance is that of an idol..." (D&C 1:16).
Because God is the source of all goodness, and because we do not merit anything of ourselves, it is death for us to take strength unto ourselves.
By describing Himself as the vine, He reminds us that we are but branches and cannot bring forth fruit of ourselves. C.S. Lewis reminds us that Satan is perfectly content to see us being chaste and self-disciplined, while he sets up in us the dictatorship of pride. He cures our cold but gives us cancer.
George MacDonald put it another way: "True victory over self is the victory of God in the man, not of the man alone. It is not subjugation that is enough, but subjugation by God. In whatever a man does without God, he must fail miserably--or succeed more miserably."