Lucifer was originally an angel, just like the others, but within him was planted this strange idea of wanting to be exactly like God, if not even better. It was pretty much jealousy which lead him to become Satan.. wanting to be God is a sin so he was condemned eternally for that.
thats the true reason:)
ok for your update question:) God never thought of us as perfect. what happened was, he created adam and eve perfect, without sin. BUT he also gave them free will to test their faith. when they were tempted by Satan, this was known as Adams fall and this is why sin now exists in the world as punishment.
then, all these other generations started emerging, and Adams sin stil remained... the only way we could be saved from sin is if God did something about it. God was the only one who could do anything since He is the only sinless one.
This is why He came down to die for all of humanity on the cross. the plan of salvation was one of ultimate love. his blood paid the price for our freedom, so that we wouldn't all go to hell even if we were righteous.
sorry if I babbled on, I hope that cleared things up a little=]