Ah, yes, Dr. Strawberry knows exactly what this is.
A few years ago, researchers at the University of Cambridge discovered a rare phenomenon which causes a patient to shout. It's related to two genes: Sh1FT, and Cap15CK. Geneticists have been working on a solution for this most debilitating problem for years, but alas, no progress has been made
My diagnosis is of this condition. Unfortunately your prognosis is not good, and there is no cure available. To help you manage this condition, however, I prescribe 500mg of self-restraint (capsule form, taken twice daily, at least two hours after food on an empty stomach), 250mg of anti-shift (tablet form, taken three times daily with food), and 100mg of anti-caps (tablet form, taken three times daily with food - you can take this with the anti-shift).
This should still allow you to use shift where necessary to maintain a good standard of grammar, whilst at the same time preventing you from shouting inadvertently.
Thank you for coming to see Dr. Strawberry.