Question

Why does the rate of a reaction increase when the concentration of reactants is increased?

Answer 1

If reactants eventually collide, there is an occurrence of reaction.

 
Therefore, when there is an increase concentration of reactant, meaning to say that there are several moles of it every unit volume. An example of this is a room having hundred of people will absolutely get higher concentration compared to a room with one individual only. 

Pertaining to effective collisions, if ever there is an increase of concentration, the frequency and rate of effective collisions among reactants surges in such a way that the rate of reaction also surges. Same with passing into a room with only 1 individual compared to hundred people blind persons, you probably want to proceed to the room with several people.

This is the simple logic behind that scientific existence.