What would be the reason that the letter 'ain has the characteristic of tawassat

Question
Assalaamu alaikum wa rahmatu Allah,
What would be the reason that the letter 'ain has the characteristic of tawassat (middle amount of timing in the running of sound) and not rikhawah or shiddah? After some tajweed studies I was much blessed to undertake, I learned and remembered the reasons for the other letters of tawassat - meem, noon, laam and ra - but I do not remember why 'ain has that characteristic.
May Allah reward you greatly in this World and the Next for the service your provide us.

Answer
Wa alaikum assalaam wa rahmatullahi wa barakatuh. 
The tawassat of the letter  is a natural end to the weak  flow of sound that the letter has when pronounced from its correct articulation point.  When a person says the letter with a sukoon preceded by a hamzah, there is a mild running of the sound, then the sound disappears.  It cannot be lengthened if pronounced correctly, even if one tries.   For this reason, it is considered a letter of the group. 
May Allah reward you likewise. 
Wa assalaam alaikum wa rahmatullah.