The avos were commanded to live in Eretz Yisroel. Today we also have this mitzvah, and therefore one may only leave Eretz Yisroel when the conditions there are unreasonable. If one simply can not make a living in Eretz Yisroel, and will have to live off of charity, and in chutz la'aretz he will be able to make a living, there is no mitzvah of yishuv Eretz Yisroel, because that is an unreasonable situation.
Likewise the Talmud says that one who wants to learn Torah in a specific yeshiva in chutz la'aretz, and will not be happy at any of the yeshivas in Eretz Yisroel, or one who fell in love with a girl from chutz la'aretz and will be broken hearted if we insist that he may not leave Eretz Yisroel, may leave for the sake of lilmod Torah or lisa isha, because the alternative is simply unreasonable.
In this week's parsha we read that Avraham Avinu sent Eliezer to chuta la'aretz to arrange a shidduch for Yitzchok. Avraham emphasized to Eliezer that under no circumstances may he allow Yitzchok to leave Eretz Yisroel. Why couldn't Yitzchok leave Eretz Yisroel? Isn't it the case that for the purpose of a shidduch one is permitted to leave Eretz Yisroel?
The explanation for this is given by Rashi in next week's sedra. Yitzchok always followed in the footsteps of his father Avraham - maintaining the masorah. The Torah tells us that he dug the exact same wells that Avraham had dug and called them by exactly the same names that Avraham had. So its no surprise that when there was a famine during the lifetime of Yitzchok, he prepared himself to go down to Egypt - just as his father Avraham had done years earlier. On his way to Egypt, Hashem appears to him in Gerar and tells him he may not go to Egypt. On the occasion of akeidas Yitzchok, Yitzchok was consecrated as a korban, and if a korban is taken outside of its "designated location" it becomes pasul. During the times of the Beis Hamikdash, the designated location for kodshei kodoshim was the azara, while the designated location for kodshim kalim was all of Yerushlayim. In the days of the avos, the azara and Yerushalayim had not been consecrated, so the designated location for all korbaos was Eretz Yisroel. Although at the time of a famine Avraham Aivnu was permitted to leave Eretz Yisroel, Yizchok, a consecrated korban, was not.
It is interesting to note that this halacha (that a korban will become pasul if it's removed from its designated location) only takes affect after shechita, which constitutes the first avodah done to the sacrifice. On the occasion of the akeida, no avodos were actually done to Yitzchok. But nonetheless, since the ram which was offered in place of Yitzchok was slaughtered, that shechita was considered as if it had been done to Yitzchok himself. Yitzchok Avinu had the halachic status of a korban on which avodos were done.
Even the avodah of haktara which was performed on the ram was considered as if it had been done to Yitzchok. Towards the end of the tochacha in parshas Behar the Torah states that Hashem will remember the covenant that He entered into with Yaakov; as well as His covenant with Yitzchok; and He will also remember His covenant with Avraham. Rashi on that pasuk quotes from the tana'im in the Sifra that the verb "to remember" only appears in connection with Yaakov and Avraham, but not in connection with Yitzchok. The reason for this is that one only has to use his memory to recall someone (or something) who is not in front of him. The avodas hahaktara that was done to the limbs of the ram was considered as having been done to Yitzchok, and Yitzchok's ashes are piled up on the mizbeach right before Hashem. Therefore there is no need to remember him.
Avraham Avinu did not misunderstand his instructions regarding the akeida. He was to consecrate his son Yitzchok as a korban olah. Under normal circumstances the avodos of any olah must be done to that particular korban, and here the malach indicated to Avraham that the avodos done to the ram will be considered as if they had been done to Yitzchok. Yitzchok was not only considered as a korban upon whom the first avodah (of shechita) had already been performed, but even the final avodah (of haktara) done to the ram was also halachically carried over to Yitzchok.