Demand curves slope (direction) because:

The correct answer to he following question is that demand curve is downward sloping, which is moving from left to right.

Explanation:

The demand curve slopes downward because of many reasons like -

1) diminishing marginal utility - according to which if as a consumer goes on consuming more and more units of product, then with each additional unit consumed the satisfaction level goes on decreasing.

2) Income effect - suppose if a persons income level increases he or she will buy more product, which means demand goes up and vice versa will also be true.

3) Substitution effect - As we know that there is inverse relation or negative cross price elasticity between substitute goods, so if price of good A increases then the Demand for good B increases.