Tar becomes glossy when its surface is smooth, fresh, warm, oily, or newly exposed. Gloss usually means light is reflecting from a continuous film of softer material.
Gloss is common on fresh tar because the surface is still smooth and rich in mobile compounds. Light reflects from it in a clean way, giving the wet black or dark brown look many people associate with tar.
Heat can bring gloss back to older tar. When sunlight warms the coating, softer material can move toward the surface and make it look shinier. A fence or roof that seemed dull in the morning may look glossier in the afternoon sun.
Gloss can also appear after abrasion or fresh wear. If the dull oxidised surface is rubbed away, darker and softer material underneath may be exposed. This can happen on rope, tools, boat parts, and surfaces that are handled or scraped.
A glossy tar surface is not automatically better or worse than a matt one. Fresh gloss may mean the coating is active and water-shedding. Persistent sticky gloss may mean too much tar, too little settling, or heat-softened material that will collect dirt.