Walk a mile in another mans shoes. Before you cast the splinter from your brothers eye, look to the plank in your own. Why did you exclude your co-worker? What was their explanation for their behaviour?
If I was the prime supporter of a system and was excluded from rewrite discussions there would be a number of things that would be going through my head. Top of the list would be "Why am I being excluded from something that everyone knows is my area of expertise, is this a move towards me being fired"? At which point emotions will quickly escalate and rational thought will take a back seat because being fired has far wider connotations than ceasing to work for a company.
If I knew that someone was the prime supporter of a system I was assigned to rewrite they would be the first person I'd include. The system has been deemed "not fit for purpose". The insights they could give might bridge the gaps between the perception and the reality. I have seen systems been declared "not fit for purpose" and projects spawned to write a replacement without there being a clear communication of precisely how the original was "not fit for purpose". This condemned the project to deliver the next generation of "not fit for purpose" software.
I agree that sulks and strops are not the way to deal with stuff but one person's sulk is another persons withdrawal to get ones emotions in check and to think things through.