Benign Prostatic Hyperplasia (BPH): Although the patient is relatively young, BPH can occur in men in their 30s. Sudden onset urinary retention can be a presenting symptom, especially if there's a significant increase in prostate size.

Urethral Stricture: This condition can cause obstructive urinary symptoms, including sudden onset urinary retention, especially if the stricture is new or has recently worsened.
Neurogenic Bladder: Conditions affecting the nerves controlling the bladder, such as multiple sclerosis or a spinal cord tumor, can lead to urinary retention. However, other neurological symptoms might also be present.
Medication Side Effects: Certain medications, like anticholinergics, decongestants, and some antidepressants, can cause urinary retention as a side effect.

Spinal Cord Compression: This is a medical emergency that can cause sudden onset urinary retention. It could be due to a tumor, abscess, or disc herniation.
Cauda Equina Syndrome: A condition where the nerves in the spinal canal are compressed, leading to urinary retention, among other symptoms. It requires immediate medical attention.
Acute Urinary Retention due to Stones: Although the patient reports no pain, a stone could potentially cause obstruction leading to urinary retention.

Bladder Neck Contracture: A condition where the bladder neck narrows, potentially causing obstructive urinary symptoms, including urinary retention.
Malignancy: Prostate, bladder, or urethral cancer can cause urinary retention, though these would be less common in a healthy 37-year-old male without other symptoms.
Diabetic Neuropathy: Although less common in a previously healthy individual, new-onset diabetes could potentially cause neuropathy affecting bladder function.