First off, if we had just overs it'd be kind of close. Maybe a fold. We also have a BDFD (nut), which is worth about 2 outs with the implied odds, however, that makes a fold here poor.

So since this isn't a terribly threatening board let's discount our 6 overcard outs to say, 4 outs. With the BDFD we're then getting full value on a total of 6 outs (which might seem like lazy math, since it'll be using the same value you were using up until this point, but it's actually about the correct number).

On the flop you're offered 7:1 to call and make a 6.7:1 draw on the next street. If you make this call you'll show a profit of about 0.05SB. If you raise you'll be paying 4:1 for a 3.2:1 draw. This play shows a profit of about 0.2SB (there's slightly more profit in drawing turn > river than flop > turn, since there's a smaller deck). Also, if you hit your heart on the turn you'll be a far happier camper having paid 2SB than 3SB to get to the river.

Now, that being said, even a passive opponent's not going to grant you a free card here 100% of the time. Sometimes he'll have a strong holding and come over the top for 3 bets on the flop, which is a disaster. This, situation, however, will be in the minority. You also have some additional implied odds on the overcards that'll outweigh the reverse by a bit (unless you play poorly post-flop) that makes this marginal looking spot quite playable.

The crux of it is that if taking a chance once is worth a certain number of units, then taking that chance twice will wield twice as many units. By raising and seeing two streets you basically make the same profitable peel twice in one hand.