Why do we yawn at Dihkr?

Peace be upon you Ô Qa’im Ale Muhammad (Qa’im of the Family of Muhammad), my question is: why man when he does Tasbih (Praise) or Adhkaar (The contemplation/remembrance of Allah) or Istighfar (forgiveness) and especially forgiveness, he starts yawning and having tears in his eyes, or also when we hear your Dhikr (remembrance of Allah) we have tears in our eyes, or when we are humble?

Enlightenment from between the hands of the Torment

The Messenger, between the hands of the Divine torment, is a Divine mercy. So how can this Mercy be distinguished? How the Messenger can be known among many false calls? How the truth can be known?!
How did the Muslims distinguish Muhammad sawas and followed him without Musaylimah or Sajjaj or Al-Aswad Al-Ansi and other Christian scholars, or Jewish scholars or Hanafi scholars?