Why did Arwen do that??

[Evisse the Blue]

It was also the Elvish (and uncorrupted Númenórean) view that a 'good' Man would or should die voluntarily by surrender with trust before being compelled (as did Aragorn). This may have been the nature of unfallen Man; though compulsion would not threaten him: he would desire and ask to be allowed to 'go on' to a higher state.

This is an ancient idea which the Oriental and eastern Europeans still hold on to but which others reject utterly as weakness or cowardice, but in fact it speaks for a higher wisdom.
It is completely opposite to the 'do not go gentle into that good night, and rage against the dying of the light' theme which seems to be very fashionable these days.
Just a thought.