Why did Snape react so angrily to being called a 'Coward' ??

[literature_Caro]

No: HPFGUIDX 134782

I am in the camp "there is at least some good within Snape and he had
a bond to DD based on true respect and freindship".

So this one and only situation (Harry chasing Snape is the only little
moment when he could live out the pain and all the guilt he felt
because within a moment he would face Voldemort and then he had to
shut them all away. And at home there is Wormtail and I really do believe that
this guy was ordered to overlook Snape and spy on him for Voldemort,
who does not trust Snape.
This pain can be seen in the HPB (ch. Flight of the Prince):
"DON'T -" screamed Snape, and his face was suddenly demented, inhuman, as though
he was in as much pain as the yelping, howling dog stuck in the burning house
behind them - "CALL ME COWARD!"

Caro