Because we are all sinners.

Romans 3:23 says that "all have sinned and fall short of the Glory of God".

Because of the fact that we are all sinners, we need a savior to save us from our sin.

Romans 6:23 says that "the wages of sin is death, but the gift of God is eternal life in Christ Jesus".

Wages, for example, are what we earn when we have a job; it is our pay. This verse says that our payment for sin -- or what we earn for sin -- is death (that is, spiritual death -- separation from God while we are still here on earth), and then eventually when we physically die, we will not be with God in heaven; instead, we'll be separated for eternity in hell.