Or HaChaim on Genesis 12:3/אור החיים על בראשית י״ב:ג׳

1 ואברכה מברכך, I shall bless those who bless you. This additional blessing was to compensate Abraham for G'd's failure to tell him the destination he was headed for. After all, G'd did not even tell Abraham which country to head for! It was a tremendous test for Abraham to just leave his home, etc., without having the slightest idea where he was to make his new home. The fact that Abraham did not query G'd in his mind or by prayer is considered as extremely praiseworthy. As a result of this blind faith in G'd, Abraham encountered success wherever he went. His spiritual stature grew constantly.

2We are entitled to ask why the wording of the Torah is different in respect to those who bless Abraham from that to those who curse him. In the former case G'd mentions that He will bless such people even before they have actually blessed Abraham. In the case of people who curse Abraham, G'd is not described as cursing such people until after they have actually cursed him. If G'd were to curse the potential curser prior to his cursing Abraham, that person would never become aware of the reason G'd cursed him. On the other hand, G'd does not cause any harm by blessing those who intended to bless Abraham already in anticipation of their good deed.

3It is also noteworthy that the Torah speaks about the people who will bless Abraham in the plural, whereas the reference to anyone who might curse Abraham is phrased in the singular.

4Perhaps this is best explained in connection with what the rabbis said to the son of Rabbi Shimon ben Yochai in Moed Katan 9. They wished him: תזרע ולא תחצד, "may you sow and not harvest," and a few more such strange sounding wishes. The Talmud describes how Rabbi Shimon bar Yochai explained to his son that though these wishes sounded like curses they were actually blessings. The reference to sowing and not harvesting was a blessing i.e. that he should father children but not have to bury them. It is not the actual phraseology that matters but the intent. G'd told Abraham that there are some people who clothe their curses in words that sound like blessings. He told him that He would also bless the individual who blessed Abraham in such an unusual manner as did the rabbis who blessed the son of Rabbi Shimon bar Yochai. The word מקללך applies both to what preceded it and to what follows it. Accordingly there are two kinds of people who might curse Abraham; the ones who dress up the curse to sound like a blessing, and the ones who curse outright. The first category is alluded to when you read the word ומקללך as if it belonged to the word מברכיך which preceded it. The other kind of curser is the one who curses without provocation and without hypocrisy. When the Torah says of G'd אאר, this refers to both categories of people cursing Abraham.

5An alternate way of explaining this is based on the statement in Taanit 20: "better the curse of an Achiyah Hashiloni (a prophet, compare Kings I 14,15) than the blessing conferred upon the Jewish people by Bileam." The former used words of a curse to give the Jewish people an incentive to overcome the curse, the latter praised them excessively, hoping to lull them into a kind of spiritual nirvana that would make them easy prey for the evil urge. Concerning the person who bestows the kind of Achiyah Hashiloni's curse on Abraham, G'd says that He will bless a person who curses with such good intentions, whereas concerning the one who does the reverse, G'd will curse him though he couched his curse in words which seemed to convey a blessing.

6ונברכו בך כל משפחות האדמה, and all the families of the world will receive a blessing thanks to you. This is the difference between people who bless you and those that are indifferent to you, neither blessing Abraham not cursing him. G'd will bless directly anyone who blesses Abraham; the ones who do not bless Abraham will receive only an indirect blessing, i.e. via Abraham.