I mean I have yet to be to a Church that does not make similar statements, they say they believe God is true and that Christ is true. Of course most acknowledged or at least to don't object if you ask them is Christ the head of there Church and are they lead be living prophets they will say NO.  
 
The Church of Jesus Christ of Later-day Saint's is lead by Christ it is Christ's true Church restored to the Earth, led by living Apostles whom are called by Christ, no other Church in the world can make such a claim.  
 
They all have dogma, traditions, and creeds to replace the Holy Apostleship, but none, not a single one can say that they are lead by a Prophet and taught by the Apostles of Jesus Christ.  
 
That is why we boldly proclaim that we know the Church is true, that it is the only Church on the face of the Earth that is lead by Christ and tended to by His Holy Apostles.  
 
We know this because we have studies the Scriptures and know that Apostasy and Restoration is a cycle of of the spiritual history of the House of Israel. That Heavenly Father calls Prophets to restore truth and over time the people reject the truth, until He sent his firstborn and only Begotten Jesus Christ, the Lord Jehovah to restore the truth in it's fullness and reestablish a true Church in the Middle East and in the New World.  
 
These restorations ultimately failed as well due to the wickedness of mankind. Our Father in Heaven though always patient and ever loving choose to restore the Church again in 1830 through the humble service of the Prophet Joseph Smith Jr. That Church has grown, faced trials, and upon reaching the promise land prospered and grew to a international membership of 14 million worldwide and growing.  
 
Members say they know the Church is true because they believe it and since they believe it they want to share the joy of that with all whom will listen!

Members who state that they know the church is true are usually not doing it to strengthen themselves. Usually it is because they have gained a testimony of the Church of Jesus Christ of Latter-day Saints and they wish to share this with others so that they too can know. 
 
In the Bible it is stated over and over again about how you can ask and actually receive a confirmation on what is true. For just a few examples see Matthew 7, John 14, 1 John 5, and James 1. 
 
If the Bible is true, than it means that you can receive answers to these questions, including which church is true. Many of these members have actually sought to know what is true because they have a desire to draw closer to their Savior. These have put Moroni 10:4 to the test and have received an answer to it and they want others to know that they can too.  
 
I have read several times in the LDS magazine about people who have gone through this. Many of them have said that something was missing in their life and they found the answers here. A lot of them talk about their confirmation of the LDS church and the Book of Mormon by the Holy Ghost.  
 
You can read many of their stories by using this address:  
 
http://www.lds.org/search?lang=eng&query=%22something+was+missing%22&collection=magazines&publication=Ensign

Of course LDS members feel that the church is true, but why do LDS need so say it so much? How come other christians of other churches dont have this tradition? Because i am sure they feel that their church is the one true church too? But they dont keep repeating it to themselves and others?

What I am talking about is not just a belief. It is a confirmation, a witness of the Holy Ghost that these things are true.  
 
This is what is different. Other churches have truth, and you can have the witness of the Holy Ghost of truth no matter where it is found. But when these people have prayed about the Book of Mormon or the LDS church being the restored church there has been a distinct external witness. It is usually not a huge thwack upside the head, but it is strong enough that you can tell that it does not originate from within you. It is a unique feeling that is always accompanied by a distinct peace. Time will erode that witness unless it is protected and cherished. If there is a God, then there is an adversary who will try to destroy that witness. 
 
It does not come to all that ask. It requires a hope that these things are true, a hope in a redeemer, and the willingness to follow through if you receive that answer. Those who read the Book of Mormon just so they can say they never received a witness will never receive that witness.  
 
Most of these people are saying these things because they have received that witness, which comes after a belief or hope. This is something that is not found in other churches, even though they may be great churches. Thus this is why you see this happen in the LDS church. 
 
I did not receive such a witness until long after I was a member of the LDS church and believed in it. It is something that I have never forgotten. This is why I know the church is true. Not because I believed it but because I confirmed it. Before that confirmation I wouldn't tell you that I knew it was true. Afterwards, I cannot deny it.

I know that the church is true, because of the doctrine and I feel it in my heart, first, this is the only church that doesn't get paid for service in church, and that includes the leaders, I don'think getting paid is right for the fact that we preach the gospel, second, if a church has the name of John is the church of John not of Christ, the true church has to have Jesus Christ name on it, and not many of all the churches has the name Jesus Christ, not many people understand this concept, third, the priesthood, not any one has the authority to baptized, or heal the sick, NOT EVERY ONE, first they have to receive this power, so that the baptism could be valid, not just faith alone will help you, their is some things that you need to do, and their is always prayer, pray that if the Book of Mormon is true, and you'll know like many latter day saints that this is His Church.