The only ones who don't claim a psychological cause are those who believe it to be physiological - i.e. a natural propensity to retain excess weight.
Well I think it is down to your genetic make-up and how that means the environment will shape that expression. I strongly believe that a more slow paced work and social life has a key part. I strongly believe that usage of anti-biotics in infants compounds the issues. I strongly believe the prevalence of poor diet in neonates and usage of formula feeds compounds the issues. I think it is a combination of physiological, environmental, and social circumstances. And I include psychological in the physiological because I don't believe in a mind/body duality and neither should you because that just turns it into a nature vs nurture debate when all sense and evidence points that it is nature shaped by nurture.