[BluffTHIS!]

You want all the money in the middle. You would rather play this headsup to avoid straight splits, and if you manage to knock out a higher flush draw then you have accomplished something important. If not, your cards lessen his flush possibilities significantly, especially if he doesn't have 1 of your straight flush outs in his hand. Another reason to get allin preflop here, is that the rock didn't reraise preflop making trip aces less likely though not improbable. So if you can knock him off top 2 of A7, and be left alone with the lag who might only have a draw, then your pair greatly increases your equity as well (or you might win it right there). Plus if the rock won't laydown and does indeed only have A7, then you would hate to fold the turn when the flush gets there fearing a bigger one or the board pairs 5s and get pushed off a draw that isn't in fact dead.