Why did Arwen do that??

[Amarie of the Vanyar]

It was also Arwen's time to die, as Mr. Underhill has so brilliantly explained.

And of course, that is completely different to suicide. Suicide is done before your time to die arrives, and it is the result of pride and despair, as Gandalf says to Denethor:

'And only the heathen kings, under the domination of the Dark Power, did thus, slaying themselves in pride and despair, murdering their kin to ease their own death'[...]
'Come!' said Gandalf 'We are needed. There is much that you can yet do.'
The Pyre of Denethor; LotR

Denethor suicides when he still had things to do, in a moment of despair (of lack of hope). Compare this to Aragorn's death. Aragorn dies when his time has come, and full of Estel. In my opinion, this two deaths are completely different, I cannot see how Aragorn's death can be compared to suicide. Neither Arwen's.