Mormon Calling And Election Made Sure Studies Doctrine Mormon
Because he is the one with the keys of the sealing power. What are you going to do? Seal yourself up unto eternal life? It can't be done.
"For I have conferred upon you the keys and power of the priesthood, wherein I restore all things, and make known unto you all things in due time.
"And verily, verily, I say unto you, that whatsoever you seal on earth shall be sealed in heaven; and whatsoever you bind on earth, in my name and by my word, saith the Lord, it shall be eternally bound in the heavens; and whosoever sins you remit on earth shall be remitted eternally in the heavens; and whosoever sins you retain on earth shall be retained in heaven." (D&C 132:45-46)
If the Prophet seals a person up unto eternal life, then they are sealed up unto eternal life by the power of the priesthood, which is the authority to act in the name of the Lord, and it is just as valid as though the Lord had done it Himself, because He gave that power to the Prophet. However, if you don't wish to believe it, that's fine. Believe whatever you want. It's fine with me.