You and a friend are trying to get things you want. You negotiate, trade and haggle...until something comes up one or both of you won't compromise on.
Substition won't work, nor will increasing what you're offering.
It might be "we do it like this or we don't do it."
There might even be another condition that gets added on that makes it even less likely the deal will "get done," especially if it's something one side can't deal with no matter what.
Eventually one side gives up and decide's it's not worth dealing now.
Congratulations, you've just seen the Deal Breaker in action.
At times it's just something small and normally people ask "Why would you not do it?" Other times it's "Well, no wonder it didn't go through" when it's something too much or too personal to do.