What are the causes of comets when it sometimes never returns?

If the assist (slingshot effect) is adequately great, the speed of the comet will be increased enough to give it escape velocity, thus changing its path to hyperbolic and it will not return. Another possibility are interlopers arriving from outside our solar system with enough velocity to not be 'captured' by the sun. read more

Comets do not produce their own light. Comets shine because of their material composition with substances like ice, and metals that reflect the light from the sun. As comets get closer to the sun during their orbits, some of that ice and rocky material streams off to form a little tail. read more

Since comet tails are shaped by sunlight and the solar wind, they always point away from the sun. Comet tails may spray planets, as was the case in 2013 with Comet Siding Spring and Mars. At first glance, comets and asteroids may appear very similar. The difference lies in the presence of the coma and tail. read more