When you are considering the odds of making your hand with two cards remaining, it is difficult to estimate the expected cost to play those two rounds. For example, if there is $6 in the pot after the flop and your single opponent has just bet $2, then your pot odds are 3-to-1. However, you have two cards to make your hand so you must try to estimate the cost of the next round. Against a single opponent the worst case is that your opponent will bet next round and you will simply call; you would be paying $6 to win $10 (5-to-3) which increases the requirement for playing.
However, since you have two chances to make your hand your potential will improve as well. If you held 4-8 and the board was 7-A-6, your estimated chance of hitting the flush or the inside straight (12 outs) after two cards is now 12/47 + 35/47 * 12/46 = .45, making it correct to call (or possibly raise) a bet on the flop.