Dear Missionaries, Joseph Smith taught:

And now, I ask, how righteousness and truth are going to sweep the earth as with a flood? I will answer. Men and angels are to be co-workers in bring to pass this great work, (TPJS 84).

The angels come down and combine together to gather their children, and gather them. We cannot be made perfect without them, nor they without us; when these things are done, the Son of Man will descend, the Ancient of Days sit; we may come to an innumerable company of angels, have communion with and receive instruction from them. (TPJS159).

The spirits of the just are exalted to a greater and more glorious work; hence they are blessed in their departure to the world of spirits. Enveloped in flaming fire, they are not far from us, and know and understand our thoughts, feelings, and motions, and are often pained therewith. (TPJS 326).

What we learn from the above statements by the prophet is that the spirits of our loved ones are very much involved in our mortal lives and ministries. President Boyd K. Packer has made it very clear, from the teachings of the Book of Mormon, that angels speak by the power of the Holy Ghost (2 Ne 32:3). They are the still small voice of truth whispering loving communications to bless, protect and save their loved ones. By and large, their ministries are neither audible nor visible. The record of sacred writ would indicate that 99.99% of their ministrations are quiet promptings of the still small voice. The word angel, like most English words, has several valid working definitions. One of the most significant working definitions of this important word is the meaning messenger. Angels are authorized messengers of salvation, by way of both invitation and warning. That's who they are and that's what they do. In this sense, missionaries are angels, for they are also authorized messengers of salvation to those dwelling in mortality. They speak by the tongue of angels, the power of the Holy Ghost, the words of Christ, which is the word of God (D&C 68:3-4). It's all about being sent (Jn 5:30; 20:21). That's what makes them angels/messengers, they're sent. They do the work of him who sends them, by the authority of him who sends them. Joseph also taught that angels and spirits have their limits and bounds:

The organization of the spiritual and heavenly worlds, and of spiritual and heavenly beings, was agreeable to the most perfect order and harmony: their limits and bounds were fixed irrevocably, and voluntarily subscribed to in their heavenly estate by themselves, and were by our first parents subscribed to upon the earth. (TPJS 325).

We see that angels cannot arbitrarily intrude in our lives. That would be a breach of our agency. There are strict bounds they cannot exceed. God's house is a house of order. These bounds dramatically limit their ministries. The only thing that can change those limitations is our faith and prayers. We must ask, and we must ask in worthiness and faith. If we do, we empower them to magnify their ministries to us, for then they are invited and even authorized to minister in our lives according to the laws and order of heaven and the eternal priesthood.

Finally, the ministry of angels protects us from evil spirits. They literally are warriors of light who ward off those malevolent beings who would destroy their loved ones. Someday, in the next life, we will come to understand how much we owe our guardian angels, it will be a sweet debt to loved ones. Joseph saw such in vision:

[Brigham Young] was preaching (to some apparently hostile American Indians) and the angel of God standing above his head, with a drawn sword in his hand, protecting him, but he did not see it. (TPJS 108).

The scriptures tell us that the angels literally have charge over us. (D&C 109:22). That is, not only do they protect and minister to us but they do so by assignment, according to the order of the patriarchal priesthood, as we have said. Joseph taught: These angels are under the direction of Michael or Adam, who acts under the direction of the Lord. (TPJS 168). The scriptures reveal that missionaries who leave their homes to serve the Lord are especially blessed with the ministry and protection of angels. (D&C 84:88; 109:22; 49:26-27; 2 Kings 6). Missionaries need to understand and believe these things with all their hearts to do their work with faith and power. This does not mean they can be careless or disobedient. To the contrary, if they are, they will not have these blessings. But if they will be obedient and exercise great faith, their ministries will be greatly magnified by the ministry of angels for it was ever intended to be a joint work. We pray you will use your co-workers more effectively by your great faith.

"And it came to pass that in this year there was one Samuel, a Lamanite, came into the land of Zarahemla, and began to preach unto the people. And it came to pass that he did preach, many days, repentance unto the people, and they did cast him out, and he was about to return to his own land.
But behold, the voice of the Lord came unto him, that he should return again, and prophesy unto the people whatsoever things should come into his heart.
And it came to pass that they would not suffer that he should enter into the city; therefore he went and got upon the wall thereof, and stretched forth his hand and cried with a loud voice, and prophesied unto the people whatsoever things the Lord put into his heart." (Helaman 13:2-4)