why reject once saved always saved?

Bible never teach that believer can lose his salvation though there are some passages that the opponents of OSAS use to support their views that salvation can be lost. They say you have to work out your salvation, you have to sort out your salvation, you have to guard your salvation. Anyway salvation is not gauranteed

What is the possible psychology behind this? My own guess is:

1. Self-righteousness. They walk in the light. They don't sin (do they?). They serve God. They attend church every sunday. They tithe faithfully. They serve in various ministries. They do good. They obey the law (all of them?). So they deserve and are qualified to go to heaven because they bear all kind of fruits.

So anyone deem (by this people) not to bear fruits will be cast into the lake of fire.

If they bear few fruits or no fruits at all (possible not to bear any fruit at all?) how can they also go to heaven also? That's not fair at all! They cry foul. God is just and fair, how can God also allow them to go to heaven!

2. Ignorant of the truth. For some reasons, they were being brought up in a church where they were taught the theory of conditional salvation. They have no assurance of their own salvation. They have to "work" to maintain their salvation.

3. They afraid that once they believe in this doctrine of OSAS, christians will become lawless and go out sin. They fear that Apostle Paul's teaching of radical grace will lead to lead to license to sin.

Where is Jesus Christ? Why are they under-estimating the power of the Father, the Son and the Holy Spirit?