"Why?" You might ask and say, "I've been baptized/saved/taken care of."
Because there is only one church with the full truth and that is His Church. The Church of Jesus Christ of Latter-day Saints.
How can I say that when there are so many good people in other churches?
Belonging to a church that doesn't have the priesthood authority to act in the Savior's name, not having the same organization, not having a living prophet to know what God wants us to do today, and more, doesn't mean that good people are bad. It just means that they belong to a very good organization that doesn't have the fullness of the gospel.
How can I say that?
Well, after Christ and the Apostles died, hundreds of years went by with no one having the priesthood authority Christ brought. Then, churches started up to try and follow Christ as best they could with many of the teachings of Christ changed. As the years went by, other churches broke off from those churches by people who didn't claim any authority, they just didn't believe their mother church was right.
That process continued down to today. If you start with no authority and modified teachings, and no living prophet to guide you, how can a church possibly maintain any type of closeness to the original Church as Christ taught?
There is the bible, and that is an amazing help. You notice throughout the bible that there were prophets all along the way until Christ. Then nothing? God didn't stop caring about His children. His children strayed. Much of the truth was lost.
Because of this straying, there had to be a restoration of the truth, the authority, and the teachings.
That restoration came when Joseph Smith saw God the Father and His Son, Jesus Christ as Joseph prayed in a grove of trees. They appeared to him after he prayed to know which church to join.
These are his words about what they said to him:
"My object in going to inquire of the Lord was to know which of all the sects was right, that I might know which to join. No sooner, therefore, did I get possession of myself, so as to be able to speak, than I asked the Personages who stood above me in the light, which of all the sects was right (for at this time it had never entered into my heart that all were wrong)—and which I should join.
I was answered that I must join none of them, for they were all wrong; and the Personage who addressed me said that all their creeds were an abomination in his sight; that those professors were all corrupt; that: 'they draw near to me with their lips, but their hearts are far from me, they teach for doctrines the commandments of men, having a form of godliness, but they deny the power thereof.'"
I know this is true. Joseph saw God, our Heavenly Father, who wants to communicate to us, and His Son Jesus Christ. Through Joseph Smith they restored the truth and His Church with the complete truth of the gospel.
That is why I think you should join the Savior's restored Church, The Church of Jesus Christ of Latter-day Saints, also nicknamed, "The Mormons."
After reading this, you may feel the Holy Ghost confirm to you that it is true. You too can find the happiness in life that you have been looking for.
For more details look at mormon.org