How can we understand the motivaton and the derivation of Thevenin theorem from scratch?
It is natural to think of constant voltage sources, since there are so many around, such as batteries and power supplies. For learning purposes, assume that they all have some internal resistance. This is the first lesson in electrical modeling: add an ideal resistor to an ideal voltage source to make a better model for a real voltage source such as a battery. 

Adding a resistor in series creates a new voltage node that is internal to the source. This new node can be inconvenient during circuit analysis. The Norton equivalent is a way to get rid of this extra node, and instead use a model with an ideal current source and a resistor. This analysis trick becomes more important when analyzing circuits in a systematic way, such as with matrix math.

For example, the voltage from a 9V battery with 36 Ohms of internal resistance can also be modeled as a 36 Ohm resistor in parallel with a 0.25 Amp current source. This makes some types of circuit analysis easier. 

If someone built a battery using a real current source this way, it would get hot. Without any load, the 36 Ohm resistor would dissipate power P = I2R =  0.252*36 = 2.25 Watts.

If someone claims that in a black box you can't tell if you have a Norton current source or a Thevenin voltage source, ask if you can have one of each, and then measure the temperature difference between them! This detail is left out of many texts, and claims of true equivalence of these circuits are misleading. Instead of motivating this theorem, perhaps it is better to demotivate it and reduce it to a circuit analysis trick.

Constant current sources are less common in everyday life, but there are some around, and they can also be translated to voltage sources for computational convenience.