In this context, for means because.
I have been playing with the words in my head, trying to delineate the exact differences between for and because. All I have come up with are
First, for has a more elegant, even pompous sound to it. The King James Version of the Bible renders Mark 5:9 as
And he asked him, What is thy name? And he answered, saying, My name is Legion: for we are many.
Most newer translations have kept the "for", but the New Living Translate goes with
Then Jesus demanded, "What is your name?" And he replied, "My name is Legion, because there are many of us inside this man."
Clearer, of course, especially to a young or ESL reader, but the power of the passage is lost.
Also, the word for -- in part, I think, because of its primary sense of purpose or intention -- carries a stronger notion of causality. "A because B" literally means "B is the cause of A" but perhaps only in a simple, unexceptional way ("I ate because I was hungry.")
"A for B" suggests "A, unlikely and unintuitive though it is, really happened and it only happened owing to B." ("I would die for her, for I love her with all my heart.")
Does that help?
"I'm not lost for I know where I am."
-- A.A. Milne
"I am determined to be cheerful and happy in whatever situation I may find myself, for I have learned that the greater part of our misery or unhappiness is determined not by our circumstance but by our disposition."
-- Martha Washington
"Yea, though I walk through the valley of the shadow of death, I will fear no evil, for Thou art with me."
-- Psalm 23:4 (KJV again)
When in doubt, use "because".