What is the importance in a mission?  Why do LDS boys ages 18-26 years of age choose to go on one?  If its for the right reasons, it is to bring the lost sheep back to the fold  Every church has similar standards and beliefs, but what about ordinances? Doctrine and Covenants , or D&C 132:7 says, "7 And verily I say unto you, that the conditions of this law are these: All covenants, contracts, bonds, obligations,oaths, vows, performances, connections, associations, or expectations, that are not made and entered into and sealed by the Holy Spirit of promise, of him who is anointed, both as well for time and for all eternity, and that too most holy, by revelation and commandment through the medium of mine anointed, whom I have appointed on the earth to hold this power, are of no efficacy, virtue, or force in and after the resurrection from the dead; for all contracts that are not made unto this end have an end when men are dead.

The gospel in the LDS church and its covenants, oaths, and laws (as said above) can bring happiness, eternal joy, and lasting relationships, if applied.  We, as missionaries, try to bring this to those that might not have these blessings in their lives currently.  D&C 131:2-4 says, " 2 And in order to obtain the highest, a man must enter into this order of the priesthood [meaning the new and everlasting covenant of marriage];

 3 And if he does not, he cannot obtain it.

4 He may enter into the other, but that is the end of his kingdom; he cannot have an increase.

 5 The more sure word of prophecy means a man’s knowing that he is sealed up unto eternal life, by revelation and the spirit of prophecy, through the power of the Holy Priesthood.

 6 It is impossible for a man to be saved in ignorance.

Marriage is the most important ordinance in the LDS church and can only be participated in by two worthy individuals, a man and a woman, that love each other.  The relationship can only be valid if the Holy Spirit blesses it, that's if the two individuals live worthy to each other.

The sealing room that couples get married in, look like this.  This is in the Payson, Utah Temple.