First, because whether you like or not by the simple fact of being a human you are sinner. (Not sure about our friends the animals)
"For all have sinned, and come short of the glory of God," Romans 3:23.
"Wherefore, as by one man sin entered the world, and death by sin; And so death passed upon all men, for that all have sinned," Romans 5:12.
Second, only through Christ you can aspire to have everlasting life.
"That whosoever believeth in Him should not perish, but have everlasting life. For God sent not His Son into the world to condemn the world; but that the world through Him might be saved," John 3:16, 17.
