When you begin with two suited cards and flop two more — meaning you need to catch one more of that suit on the 4th (turn) or 5th (river) cards, it's often correct to continue to pursue the pot. So, if you start with Kh-Jh in a fixed-limit game and the flop is 7-A-4, you probably should call (and sometimes even bet, partially for deceptive purposes).
But if there's a pair on that flop, such as 10-10-7, you should often fold.
It's because the flush attempt is usually only marginally profitably (on average) without the pair present. At PartyPoker he increased chances of you making a flush only to have it beat by a full house when a pair is present often makes the pursuit unprofitable.
That's why, if you think the pursuit of the flush would only show a small profit without the pair, you should almost always fold with the pair present. Rare exceptions might involve times when you can use the presence of the pair to bluff or to posture.