Tar is sticky because it contains heavy oils, resins, and complex organic compounds that cling to surfaces and do not evaporate quickly.
Stickiness is one of tar’s defining behaviours. Unlike water or alcohol, tar is made of many heavier compounds that move slowly and grip surfaces. Some parts remain oily or resinous. Others thicken as lighter fractions leave. The result is a material that can cling, string, smear, and penetrate.
The original feedstock matters. Pine tar inherits some character from resinous wood. Birch tar is known as a strong prehistoric adhesive. Coal tar and bituminous materials contain heavier aromatic and hydrocarbon fractions. Different kinds of tar can therefore feel sticky in different ways.
Stickiness made tar useful. It helped tar stay on rope, timber, seams, and outdoor surfaces. A material that washed off immediately would have been far less valuable. The same property also makes tar annoying: it stains clothes, traps dirt, holds grit, and can be difficult to remove from tools and skin.
A tar that stays sticky forever may be overapplied, unsuitable for the surface, too cold, poorly dried, contaminated, or simply the wrong product for the job. Stickiness is useful, but controlled stickiness is the goal.