final -s, IE

From: Joao
Message: 34643
Date: 2004-10-13

Is there any explanation why some IE languages dropped -s and another didnt?
Ending -S is kept in:
Baltic
Latin (even Romance)
Proto-Celtic, Gaulish
Greek
Hittite
 
Ending -S is dropped in:
Germanic
Slavic
Sanskrit (>h)
Avestic
Armenian
Modern Celtic languages
Tocharian
 
 
Is there any reason to drop the -s ? Accent  position? Substratum? The presence or ausence can be used to define some kind of group?
 
Joao SL