I'm not so sure. I've had my share of severe stress in my life, and I'm of shy, slightly nervous, and introverted disposition; but so are my mum and 2 sisters who all lived through those years of hell with me. My mum suffers from severe depression, anxiety and phobias/panic attacks and my sisters are of similar disposition to me but.... I'm the only one with IBS.
I'm convinced IBS has a physical cause somehow, but that stress is THE major trigger that can tip you over the edge into life-interfering level of illness.
Also, there are many people with healthy, happy backgrounds who IBS hits quite early in childhood or much later in life. We don't all fit the same pattern.