In the fic, Castiel was an alpha while Dean was an omega (I’m pretty sure). I remember that Castiel was afraid that he might harm Dean because of an incident in his past. 

The incident was that Cas got into an elevator with an omega (I think Alfie, Inias, or an OC) and almost attacked him due to rut or something of that nature. But, Castiel stopped himself (due to seeing the supposed fear in the omega) and got out of the elevator before he could do any harm. Thus, Castiel got super fearful of harming omegas and tried to gain high levels of self control as a result. I also remember that it was found out later that the omega actually was trying to attract Castiel in the elevator and wasn’t actually afraid.