I am studying about Hilbert Tranform in SSB modulation for AM

1) When I read about matlab docs on hilbert transform , I am bit confused on why "a signal's negative frequencies are phase-advanced by 90 degrees and the positive frequencies are phase-delayed by 90 degrees." ?

2) For hilbert transform of cosine function , why "The area of the impulse at wc changes from pi to -j*pi, while the area of the impulse -wc changes from pi to j*pi" ?

3) I believe ideal hilbert transform is not realizable, why so ? Note: There are already algorithms that approximate the Hilbert Transformer, such as the Parks-McClellan FIR filter design technique