Costochondritis: This condition, also known as Tietze's syndrome, is characterized by inflammation of the cartilage that connects the ribs to the sternum. The symptoms of costochondritis, such as tenderness to the left of the sternum, pain upon coughing or deep breathing, and the absence of systemic symptoms like fever, closely match the patient's presentation. It is a common cause of chest pain in young adults.

Musculoskeletal strain: Given the location and nature of the pain, a musculoskeletal strain or injury could be a plausible explanation, especially if the patient has recently engaged in unusual physical activity.
Gastroesophageal reflux disease (GERD): GERD can cause chest pain that may be exacerbated by deep breathing or coughing due to the irritation of the esophagus. However, the pain is typically more retrosternal and may be accompanied by symptoms like heartburn.
Precordial catch syndrome: This is a benign condition that causes sharp, stabbing chest pain, often precipitated by movement or deep breathing. It can occur at any age and may be related to the musculoskeletal structures of the chest wall.

Acute coronary syndrome (ACS): Although less common in a 27-year-old male without risk factors, ACS (including myocardial infarction) must be considered due to its potential for severe consequences. The absence of typical risk factors or symptoms like arm pain, jaw pain, or shortness of breath does not entirely rule out ACS.
Pulmonary embolism (PE): PE is a life-threatening condition that can present with sudden onset of chest pain that worsens with deep breathing (pleuritic chest pain). The absence of fever and dizziness does not exclude this diagnosis, and any suspicion warrants immediate further investigation.
Pneumonia or pleuritis: Infection or inflammation of the lung or pleura can cause chest pain that is worse with coughing or deep breathing. Although the patient does not have a fever, these conditions should be considered, especially if there are any respiratory symptoms.

Sickle cell crisis: In patients with sickle cell disease, a crisis can cause chest pain due to vaso-occlusion. This would be an important consideration in patients with a known history of the disease.
Tietze's syndrome variant involving other areas: While costochondritis typically involves the second or third costosternal junctions, Tietze's syndrome can involve other areas and present with similar symptoms.
Other inflammatory or autoimmune conditions: Conditions like rheumatoid arthritis or lupus can cause musculoskeletal pain, including chest wall pain, though these would typically be accompanied by other systemic symptoms or a known history of the disease.