Some more details need to diagnosis.
Distant reasons are related to food/ sleep/ work habit of an individual.
Bodily reasons are related to physiological factors. May be a degenerative / inflammatory / obstructive in nature.
Next Steps
Pain in left side of chest
may be a simple digestive system origin.
or
may be related to a organ in thorax cavity like lung/ heart/ diaphragm.
Health Tips
Better to consult a Doctor directly.