Why are individual market supply curves, such as the supply of accountants, upward sloping, or positively sloped in microeconomics?

This is more a question of rational behavior of people who
have taken up different professions like accountancy, etc. If each accountant is being
offered more money to do the same work, an increased number of people will offer their
services as an accountant.

And this is true of almost every
profession. If the wages of janitors were to become equal to that of lawyers, I do not
think there would be a shortage in the number of people who would want to become
janitors, with even those who are presently lawyers willing to abandon their professions
and become janitors.

An increase in price
leads to an increase in supply.