Sforno on Genesis 26:5/ספורנו על בראשית כ״ו:ה׳

1עקב אשר שמע אברהם בקולי, in accordance with all that I commanded him;

2וישמור משמרתי, he also did the very things which I Myself am in the habit of doing, such as to perform deeds of loving kindness. (compare Psalms 25,10 כל ארחות ה' חסד ואמת, “all the paths of the Lord are love and truth, etc.”) Also, Avraham warned the potential sinners to improve their ways, something that is of utmost importance to Me. This is what he did every time the Torah records him as קרא בשם ה', “he proclaimed the name of the Lord.” He also observed My revealed commandments מצותי, חוקותי, ותורותי, “the seven laws I laid down for all of mankind.” As a result of all this, he did not only preach to his fellow man but he practiced what he preached. Due to his possessing this virtue he became a model human being for others to emulate. At this particular stage in Yitzchok’s life, G’d did favours to him which were the result of the merits of others, i.e. the merit of his father. This theme is repeated once more in verse 24 when the Torah attributes the promise of the increase in Yitzchok’s seed to the merit of Avraham, i.e.בעבור אברהם עבדי “for the sake of My servant Avraham.” Neither Yaakov nor Avraham had ever been told that G’d’s promises to them was on account of a third party’s merit. The reason G’d’s promises to Yitzchok had been due to other people’s merits was that at that time he had not yet emulated his father’s practice of proclaiming the name of G’d, i.e. calling his fellow man to improve their lifestyles and to abandon idolatry. Once Yitzchok had begun to do this we find that even an Avimelech is in awe of him and recognises that he is the recipient of G’d’s blessings in his own right. (compare verse 26, as well as verses 28-29. All this was due to the fact that in verse 25 he is reported as emulating his father, building an altar and proclaiming the name of the Lord. Yaakov, as opposed to his father Yitzchok, has been described as being a יושב אהלים from his earliest youth, i.e. devoting himself to promoting matters spiritual, studying as well as teaching in the academy of Shem and Ever, (primarily those of the seven Noachide commandments which regulate inter personal relations.)