If you have transferred your text messages from an "old" phone to your new Samsung Galaxy S6, then it can happen that some of these text messages suddenly disappear or get deleted. The function that is responsible for that on the Samsung Galaxy S6 is: "Delete old messages".
Here the oldest messages are deleted when the maximum number of SMS (1000) and MMS (100 pieces) is exceeded. Since you have most of the times stored 1000 text messages ore more on your Samsung Galaxy S6, it can easily happen that you are now faced with the accidentally disappearance of your messages.
Unfortunately, you can’t retrieve these messages again, but at least the function that is responsible can be disabled on the Samsung Galaxy S6. To change it, navigate to this page:
Menu --> Settings --> Applications -->Messages -->More Settings
Disables the function "Delete old messages" now on your Samsung Galaxy S6. Finished! Immediately your text messages will not be deleted or will disappear on the Samsung Galaxy S6.