The Reason Why
Because he remembers…and he remembers it's his fault.
Because he should have died, and yet he didn't.
Because he tried, but he couldn't fix it.
Because Dean needs his brother more than anything.
Because he knows that Dean would always choose Sam.
Because he knows Dean's choice would be right.
Because he never wants Dean to have to choose.
...and Dean's needs always come first.
A/N: In case it wasn't entirely clear, this is meant to be set during 7.17, and "he" is meant to be Castiel.