At the beginning of scene one, we find Benvolio and Mercutio in the town square. They are basically debating whether to go home, as the weather is hot and people's moods are hotter. And Benvolio, at least, is concerned that a fight, given the proper circumstances (i.e. any circumstances at all) Mercutio will end up fighting. Apparently Mercutio fights because he enjoys it. He chooses based on some incentive system – he may have a death wish, he may be an adrenaline junky, he may have yet to lose a fight and thus have an unrealistic view of his own mortality (contrary to the first idea).
Tybalt then appears in the square with some of his homeboys. Tybalt seeks Romeo to settle some imagined slight. Romeo arrives but declines to fight. But Mercutio gives in to his value system and draws on Tybalt. Soon after, the incentive of death to the head of house is forgotten (even though Romeo reminds them) and fight ensues. First Mercutio is killed as Romeo chooses to intervene and prevent bloodshed. Tybalt runs away, but returns to seek Romeo. But with the death of his friend Mercutio, Romeo's incentive structure is now changed. He fights and slays Tybalt. Romeo then flees. This leaves Benvolio to explain all to the Capulets and the Prince. The Prince, facing a cost from a previous choice, changes the rules again, deciding enough blood has been shed and nothing may be gained by taking the heads of the feuding houses. Thus he chooses to banish Romeo. This scene offers many examples of people choosing, (to not fight or to fight), and of people responding to incentives, (whether it be an ineffectual incentive – the death of someone else, or a more effective incentive – revenge). And the choices have costs. For Mercutio, the decision to pick a fight cost his life. For Tybalt, the choice of returning to look for Romeo cost his life. And for Romeo, the choice of fighting and killing Tybalt has cost him his citizenship to Verona. All costs were in the future (remember, the Prince has chosen not to enforce a previous decision against fighting – perhaps the costs of the previous choice were too high - and I suspect none of the fighters thought they would lose). And we've not yet tallied all the costs of Romeo's choice, as we shall see.
We now go to Capulet's orchard, the setting for scene two. Here, Juliet awaits her husband, but instead is visited by her nurse who brings word of the death of Tybalt (Juliet's cousin) at the hand of Romeo and Romeo's subsequent banishment. At word of this, Juliet must choose where her greater allegiance lies – with her old family or her new. Juliet chooses, and again, the cost of that choice lies in the future.
We find Romeo at Friar Lawrence's in scene three. Here, upon finding he's been banished to Mantua, Romeo bemoans his fate – one worse than death in his eyes. What appears to be the full future cost of his decision to kill Tybalt is now apparent to him. He must live without his Juliet. Juliet's nurse brings news of her mistress's distress (now calling Tybalt, now Romeo), which further upsets Romeo. But the good Friar makes Romeo see all is not lost. The Friar proposes Romeo go to Mantua and he (the Friar) will try to get the Prince to rescind his choice banishing Romeo, while informing the feuding families of the marriage. And at some time in the future, Romeo will be able to return. Thus, if Romeo chooses to accept banishment, the immediate future cost of the choice is life in Mantua. The incentive is life with Juliet.
We now head back to Capulet's house for another short scene, a meeting between Lord and Lady Capulet and the suitor, Paris. Here, Lord Capulet assures Paris that Juliet's choice will be moved by her father's judgment, and the wedding bells will ring sometime soon. Lord Capulet is confident that his judgment is sufficient incentive for his daughter on the question of marriage. The "cost" of that choice is a wedding in the future.
Scene five is the last scene in this act. It opens with Romeo and Juliet awakening after their wedding night. They debate whether various portents are of the night or the morning. But Nurse arrives and removes all doubt. It's morning and Juliet's mom is on her way to visit her daughter. Romeo leaves, and Juliet's mom arrives. She informs Juliet that marriage to Paris is in the offing. Juliet will have none of that. Lord Capulet then arrives and finds that his judgment is not sufficient incentive for Juliet to choose Paris. (Of course, he doesn’t know she's already married – an institution that, along with love of Romeo, presents a large incentive to choice.) Lord and Lady Capulet then place a new cost on Juliet's choice to reject Paris. They will, in the near future, turn her out of the house. (This actually presents a positive rather than negative incentive, as it would free Juliet to join Romeo in nearby Mantua.)
As Act III ends, people are choosing and offering incentives with costs in the future all over the place. This play is becoming a playground for studying choice and decision-making.