I don't think I have the emotional range to "get angry" the way most people describe it (as some overcoming urge). It's an alien concept to me. For me, anger is a quiet loosening of my moral obligation towards someone, a re-tallying of social contracts, something done consciously and with purpose.
If I should appear angry, but just "go with the flow" instead, it doesn't mean I'm not angry -- it means I no longer feel the need to be honest with you about my thoughts or feelings. I've found that by and large, people fail to notice the difference.
So if it is any consolation, at least some of us who appear easygoing are actually furious internally.