Why do we assume Kinetic friction to be constant? My Physics teacher told me that friction exists because of bonds made between the surfaces of two objects. As we try to move an object, we break the bonds and this causes release of energy which is heat. He told us that Kinetic friction is smaller than static friction because when we move an object, it doesn't get much time to make strong bonds! 
My question is, if such things happen in real world, then as we increase the speed of the object the surfaces would get lesser and lesser time to make bonds and friction would decrease . But this doesn't happen. Why? 
 A: The Laws of Dry/Solid Friction - such as that the friction force is independent of area of contact, proportional to the normal force between surfaces, and is independent of their relative speed - are empirical laws based on observation, like Hooke's Law and Ohm's Law.  They are generally and approximately true in most situations for relatively small forces and speeds. Various theories are put forward to explain them, such as the making and breaking of bonds ('stick/slip'). But that is only one of several processes which are involved.  Real surfaces also have small amounts of contaminants such as oil or grease which provide lubrication and complicate the situation.
Like all empirical laws they break down in more extreme situations or when tested with greater accuracy.  As the speed of relative motion increases the friction force does in many cases decrease as you suggest it should, because of heating or aging.
We assume the Laws of Friction are true because they are useful, convenient approximations.  They enable us to calculate and predict approximate analytical results without too much effort, especially when our object is to learn how to develop and use models to solve problems, rather than to model a real situation accurately.  When accuracy does matter, more realistic empirical laws are used instead, and because of the complexity of these laws numerical calculations have to be made.   
https://hal.archives-ouvertes.fr/ensl-00589509/document - courtesy of AnnaV
https://arxiv.org/abs/cond-mat/0506657
A: There are two reasons for friction:

*

*Interlocking

*Bonds or cold welding

When an object is at rest it makes bonds with the surface and the irregularities on the surface of both of the objects get interlocked due to which the friction at that time is called static friction and has the highest friction force. But when you apply force and make the object move then the bonds between them break down and the friction now is called kinetic friction which is due to only interlocking between the surface of two objects. The kinetic friction formula is normal × friction coefficient (which is constant) so if you increase the force you don't change the normal force due to which the friction remains constant.
