Supply curves are traditionally shown on a graph as sloping upwards from left to right. There are a number of reasons for this. First and foremost, there is the profit motive. An increase in demand for a particular product in the market will cause the price of that product to rise. In turn, this will encourage firms to increase their output in order to make more profit, thus increasing the level of supply. Such a development is illustrated by the upward slope of the supply curve on a graph.

Once the production of a specific good has increased, in most cases this means an increase in the costs of production. It's not hard to see why. If firms increase their production, then of course it will cost them more to produce additional units. Inevitably, this additional production will increase supply, which, as we have seen, is illustrated on a graph by an upward sloping curve.

The prospect of increased profitability will inevitably tempt new entrants to come into the market. As they enter, they will produce more of the goods that can command a high price in the market. The main consequence of this additional production will be an increase in the overall level of supply. And as we've already seen, this increased level of supply is illustrated on a graph by an upward sloping curve.

The supply curve slopes upward due to the value of the commodity and the inherent profit a manufacturer or supplier would receive for supplying said product. The supply curve is bounded by quantity supplied (x-axis) and price (y-axis). As the price increases for the product, so does the potential for profit (assuming everything else about the product remains the same—cost of production, demand, etc).

As a result of this increase in potential profit, it is more valuable for a supplier to produce or supply this commodity, because they will receive more money per unit supplied. Because of this, they will inherently create and furnish more of that good in order to profit off of it. In a straightforward economy (perfectly competitive), all firms would react this way and would produce more under these circumstances.

The primary reason the supply curve slopes upward is because of potential for profit. Essentially, the supply curve illustrates how much a firm is willing to sell or supply, based strictly on firms in perfect competition. If a firm is in perfect competition, it will follow equilibrium principles and will be taking simple, economic steps towards making a profit.

The higher the price of a product—or, more accurately, the more a customer is willing to pay for it—the more incentive a producer has to supply the product, because they will receive a higher profit. Because of this, the supply curve will necessarily increase (quantity supplied will increase) in direct relation to the increase in prices of a specified good, in order to profit the manufacturer the most.

The supply curve slopes upwards because suppliers are motivated to increase supply when the price is high—a principle of profit maximization. Higher prices result in higher revenues for suppliers, which helps them meet the costs associated with running the business while making higher profits. However, it is important to note that higher prices will reduce demand except for products and services that customers cannot do without. In such a situation suppliers increase the supply, but the customers will not increase the demand. Thus, efficiency is achieved at the point (equilibrium) where the demand curve and supply curve intersect. The supply curve will also slope upwards because suppliers are trying to make a return on their investment. They will put their equipment and resources into effective use to ensure they produce more and reach many customers to increase their revenues.

The basic reason why a supply curve slopes upward is because producers want to make as much profit as they can.  The fact that the supply curve slopes upward means that there is a direct relationship between the price at which the producer can sell their product and the quantity of the product that they can and will sell.  This makes sense because a higher price will bring (all other things being equal) a higher profit.   As the sale price of the good rises, the profit gained by selling it increases and the producer has a greater incentive to produce and sell a higher quantity.

 Another way to look at this is that firms have to keep their marginal revenues equal to their marginal costs.  This is an economic law because that is the way to maximize profit or minimize loss.  Because of the law of diminishing marginal returns, marginal costs tend to increase as a firm produces more of a thing.  When this happens, firms will need to charge higher prices so that their marginal revenue can continue to equal their marginal costs.

Thus, we have two main ways of explaining why a supply curve slopes upward.