Is there a mitzva to be a “good person”?
One of the general commandments in the Torah is Ve-asitah ha-yashar ve-ha-tov lifnei haShem – that you should do what is right and good before God (Devarim 6:18). This mitzva is understood by the Sages as requiring the Jewish people to behave appropriately towards others even when there is no specific monetary obligation to do so. One example is the rule of bar mitzrah – a neighbor. Someone who owns an adjoining field has the first rights to purchase it in the event that his neighbor decides to sell it. Since there are obvious advantages to owning two fields that are right next to each other, the Sages established a number of enactments that give the neighbor preferential treatment when the field is being sold.
The Gemara also enumerates a number of cases where the laws of bar mitzrah do not apply. Thus, someone who sells all of his possessions to a single individual will not have to offer first rights of refusal to his neighbor. (Some say that this is because he may be allowed the convenience of selling everything at once, and not having to worry about multiple contracts and receipts. According to the Rambam this is because someone who sells all of his possessions must be forced to do it because of pressing family or financial reasons, and we will not force him to begin negotiations with the neighbor.) Similarly someone who sells back to the previous owner – i.e. the person he bought it from – does not have to offer it to the neighbor, or if he gave the field as a present to someone, he does not have to offer it to the neighbor.
A notable exception is someone who purchases a field from a non-Jew, who can tell the neighbor that he should be thanked for “chasing away a lion,” i.e. that he should be happy to have a Jewish neighbor (who will work out disagreements through the Jewish courts, etc.) rather than a non-Jewish one. The Rosh challenges this Gemara by asking why this is such a good argument – shouldn’t the purchaser still be obligated to offer the field to the neighbor based on the commandment ve-asitah ha-yashar ve-ha-tov? This leads the Rosh to conclude that only the seller is obligated in the laws of bar mitzrah. The purchaser has no responsibility at all to make sure that the neighbors are pleased with the sale.