1) make it more than 3x when the SB open-completes
2) Villain tendencies, reads, anything? VPIP/PFR? Loves to chase flush draws? Likes boobies?
3) Pot limit holdem?
4) Punch your monitor as punishment for the flush getting there, then yourself in the face. Why are you playing 25PL? Seems wrong.
Edit: put your opp on a range; generally we should take a bet/fold line on turn / river here since a lot of worse is calling and (mostly) only better is raising.