I would like to give my opinion with limited knowledge that i have & without insulting any shia that do matam:
Fair enough, you remember that Huseyn as was martyred in Muharram, but so do sunnis but we don't beat ourselves like some shias do. Doesn't islam tell us to not harm our body as it is "imanat"/gift of Allah? Secondly why not just cry, or do zikr instead of beating ourselves? Thirdly what happens if someone dies from beating himself so hard? would you count him as a suicide or accident?
very reasonable questions.
First of all we do not believe that beating oneself is obligatory or wajib. The question is only the allowability. Many people cry in the majalis, many do not. Many just get up and leave after the majlis while many stay behind to do the matam. Many take it further and shed blood, large majority do not.
2nd- Body indeed is a gift of God but remmeber that soul is superior to the body. Soul expresses emotions through the body. And if these emotions are for a Just cause and for Allah (swt) then is it still wrong? Reflect on the following verses:
"Then came forward his wife in grief, she smote her face and said (what! I) An old barren woman?"
Quran 51:29
Also Hz Yaqoob (as) cried so much for Hz Yusuf (as) that he lost his eyes. Was he doing injustice to his body, nauzobillah?