The first contributor to the problem is that when someone first comes to faith in Christ and is born again, they do not see themselves as "babes" in need of the "milk" of the Word, and they are not taught it! First Peter 2:2 says, "As newborn babes, desire the sincere milk of the Word, that ye may grow thereby." Apparently, Pastors are far more interested in getting people saved, than they are in equipping them to reach the lost world for Christ, even though the Word of God says that is their job (Eph. 4:11-16).
A second contributor is that new Christians are not taught that even though they are saved by faith in Jesus Christ, they will still struggle with temptation due to their own sin nature. Romans 7:14-25 describes the Apostle Paul's on-going battle between the Spirit who dwells within him, and his flesh. Born again believers still sin! That is why John wrote 1 John 1:8-10, and James wrote James 5:13-16. And it is always when we, as believers, sin, that we begin to doubt our salvation!
There may be more factors that contribute to one doubting his or her salvation, but the only other one I can think of, is a faulty interpretation of Bible passages that appear to teach one can lose his salvation. The Bible does not contradict itself, so we are either sealed by the Holy Spirit unto the day of our redemption and are His (Eph. 1:13; 4:30), or we need to live a life of fear and legalism.
The Word of God says, "These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God" (1 Jn. 5:13). It also says, "Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when He shall appear, we shall be like Him; for we shall see Him as He is" (1 Jn. 3:2).
Do you believe that Jesus is the Christ of Israel? If so, you are saved!