unless you hold the supreme ultimate nuts every hand you can say "well he's either ahead or behind so why make it easy for him to decide?" every time. poker is about cashing in on those mistakes people make and putting in raises with the most likely best hand is how you do that. sometimes you are behind, but you don't want to minimize every winning hand you have just because you think the opponent knows how to play perfect poker. if you think you could have folded to the reraise then why not do that instead of calling down? probably costs the same and on top of that you don't have to worry about the 4-flush hitting or the board pairing. people will raise your flop bets with all sorts of shit so I wouldn't slow down my 2card flush every time someone does that unless you have a read.