Tar sticks to surfaces through wetting, mechanical grip, viscosity, and gradual setting. It bonds best to clean, dry, porous, or slightly rough surfaces.
Tar is naturally adhesive because it is thick, oily, and able to wet many rough surfaces. When it touches wood, rope, stone, or old porous material, it can flow into tiny irregularities. Once it thickens and weathers, those irregularities help hold it in place.
Surface condition controls much of the bond. Tar does not stick well to dust, loose old paint, wet timber, frost, oil, wax, or polished non-porous surfaces. It may appear to cover them at first, but the weak layer underneath can fail later.
Temperature changes the sticking behaviour. Warm tar flows and wets surfaces more easily, but it can also remain tacky and pick up dirt. Cold tar may be too stiff to make good contact. A coating can fail simply because it never properly reached the surface beneath it.
Tar adhesion is therefore a mix of chemistry and texture. It sticks because it grips the surface and then becomes less mobile, not because it forms one universal permanent glue on everything it touches.