Consider 2 scenarios, in which a flop has given you an approx. 33% chance of making an Ace high flush by the river. The pot size is currently 600 and there is 1 player remaining. It costs 200 to call.
Suppose you call and your opponent after the turn card (which doesn't make your flush) bets 200. The pot size is now 1000 and it costs 200 to call. With odds of 1:5 and a 20% chance of hitting the flush on the river it makes sense to call.
In this scenario, your initial call of 200 after the flop had a positive EV.
Again you call the bet of 200 after the flop, and again the turn card doesn't give you your flush. Now your opponent is a very aggressive player and bets 1000 chips, bringing the pot size to 1800. This gives you odds of 1:1.8, which is definitely not enough to call on a 20% chance of making your flush on the river. You must fold.
Now in this scenario, it was only correct to call the initial bet of 200 on the basis that you will see 2 more cards. However against such an aggressive player, it can be shown mathematically that calling after the flop in this situation will yield a long term net loss, since 80% of the time you are going to fold on the turn, only seeing 1 card, not 2 which the pot odds were calculated for.
The size of the pot required to justify chasing a flush after the flop depends on how likely your opponent is to let you see both the turn and the river. If you are against a new opponent and you do not know whether the opponent likes to bet big on the turn or just call or min bet, what pot size do you really need in order to chase your flush?