The Oxford English Dictionary and most scholars state that sincerity from the root sincere is derived from the Latin sincerus meaning clean, pure, sound. There is also an often repeated folk tale about how sincere is derived from the Latin sine = without, cera = wax. According to one popular explanation, dishonest sculptors in Rome or Greece would cover flaws in their work with wax to deceive the viewer; therefore, a sculpture "without wax" would mean honesty in its perfection.
Either of these word origins is fine for our purposes. Another dictionary definition lists sincere as free from pretense or deceit; proceeding from genuine feelings.
In Doctrine and Covenants 121:42 it says that the priesthood should be used without hypocrisy. Hypocrisy is defined as the practice of claiming to have moral standards or beliefs to which one's own behavior does not conform.
There is an important distinction between a person with hypocrisy and a person who has set high moral standards for themselves and occasionally falls short. Like most things related to the topic of leadership it all depends on where your heart is. A hypocrite does not hold the moral values in his heart, but only claims to have the values without living them. If, in your heart, you have beliefs that don't always match your actions, then you are not being hypocritical just a flawed human or in other words you are succumbing to the natural man (Mosiah 3:19). I think this is what is meant in Moroni 10:4 when it talks about having a sincere heart with real intent. Leading with love means leading with your heart. If your heart is out of alignment, then your leadership will suffer. Leaders must have a sincere heart without hypocrisy.
Consider this quote by Elder David A. Bednar,
"Many of the Savior’s harshest rebukes were directed to hypocrites. Jesus warned His disciples concerning the scribes and Pharisees: “Do not ye after their works: for they say, and do not” (Matthew 23:3). This strong admonition is sobering given the counsel to “express love—and show it,” to “bear testimony—and live it,” and to “be consistent.”
"The hypocrisy in our lives is most readily discerned and causes the greatest destruction within our own homes. And children often are the most alert and sensitive when it comes to recognizing hypocrisy.
"A public statement of love when the private actions of love are absent at home is hypocrisy—and weakens the foundation of a great work. Publicly declaring testimony when faithfulness and obedience are missing within our own homes is hypocrisy—and undermines the foundation of a great work. The commandment “Thou shalt not bear false witness” (Exodus 20:16) applies most pointedly to the hypocrite in each of us. We need to be and become more consistent. “But be thou an example of the believers, in word, in conversation, in charity, in spirit, in faith, in purity” (1 Timothy 4:12).
"As we seek the Lord’s help and in His strength, we can gradually reduce the disparity between what we say and what we do, between expressing love and consistently showing it, and between bearing testimony and steadfastly living it. We can become more diligent and concerned at home as we are more faithful in learning, living, and loving the restored gospel of Jesus Christ. (David A. Bednar, "More Diligent and Concerned at Home", October 2009 LDS General Conference)
Cross references for additional study: