Short-faced dogs breathe audibly, and the sound is frequently described as characteristic of the breed. It is a description of restricted airflow rather than of personality.
The mismatch between bone and tissue
Selective breeding shortened the facial bones substantially over a relatively small number of generations. The soft tissue within the head did not reduce proportionally.
The same volume of tissue therefore occupies a much shorter space, and it occupies it by folding, thickening and protruding into the airway.
The result is a passage of reduced cross-section along most of its length, from the nostril to the entrance of the windpipe.
The specific narrow points
Nostrils are often narrowed to slits, which restricts flow at the entrance and increases the effort required for every breath.
The soft palate is frequently too long for the shortened skull and extends into the airway, where it vibrates and can be drawn across the opening during forceful inhalation.
The turbinates inside the nose remain full-sized and become compressed and folded, obstructing a passage that is already reduced.
Why the problem worsens itself
Breathing against resistance requires greater negative pressure, and that pressure pulls soft tissue further into the airway.
Over years this stretches and thickens the affected tissue, and secondary changes develop in structures further down, including the larynx.
The condition therefore tends to progress rather than remain stable, which is why signs often become more noticeable in middle age.
Heat is the acute danger
Dogs shed heat primarily by moving air across moist surfaces in the mouth and airway, which depends on being able to move a large volume of air.
An affected dog has both a reduced capacity to do that and a higher heat load from the effort of breathing itself.
Overheating in these breeds can develop quickly in conditions that other dogs tolerate, and it is a genuine emergency requiring immediate veterinary care rather than home management.
What owners can influence
Excess weight adds tissue around the airway and increases oxygen demand, and weight control has a measurable effect on how affected dogs cope.
Harnesses rather than collars avoid pressure on an already compromised airway, and exercise timed to cooler parts of the day reduces the load considerably.
Snoring, gagging, exercise intolerance and blue-tinged gums are signs to have assessed rather than accepted, and surgical options exist for several of the components. Whether any apply is a veterinary judgement for the individual dog.