yeah ! its a good question :) here : Allah Said : تَحِيَّةً مِّنْ عِندِ اللَّهِ مُبَارَكَةً
"there is barakah on salam"
and salam is du'a .. Islam is Rahmah lil 'Alamin, so can you imagine, how pretty beautiful muslims keep du'a" peace be upon you" each other.:)
so, hello and hi or whatever .. those cant make du'a. they're nothing !
so, lets just make du'a each other . ISLAM PEACE :)