why do frictional force travel in the direction opposite to the body?

Frictional force opposes the relative motion between two surfaces in contact. 

Surface of a moving body appears smooth by naked eye, but when observed through microscope it is found that all the surfaces have irregularites.

The frictional force between two surfaces causes due to the interlocking between the irregularites on the contact surface.

Friction causes wear and tear of surfaces in contact of moving parts. 

A body sliding on a surface comes at rest after covering finite distance because of frictional force.

 

Hence we can say that frictional force always acts in the opposite direction of motion and tries to slow down the motion of a moving object due to the interaction between to surfaces.