Why does the rate of reaction increase when substrate concentration increase?

well easily explained..if you are really hungry and you got baskets and baskets of oranges, you will obviously keep on eating. Same with enzymes, when they are in ideal temperature (hunger) and they have an good supply of substrate (oranges) they just keep on going. respiration gives out heat as well, so when substrates are broken down they give out heat an give more energy to other enzymes making them gain activation energy and work more efficiently too.