The bible teaches that the believer is secure in Christ Jesus until the day of redemption. Salvation is a work of God, not man. Man plays no part in salvation other than receiving Christ. It is God who saves and it is God who keeps a person saved. A person cannot save themselves and a person cannot keep themselves saved, no matter how hard they try in their own human efforts.
Also, a person who believes that salvation can be lost cannot currently say that they have eternal life, and they cannot currently say that they will one day be with the Lord because they have no way of knowing. If a person thinks they can lose salvation, how can they even know they have salvation now? What joy can a believer have if they cannot know for certainty that they will one day be with their Lord and Savior?