So I despised life, for I was depressed by all that goes on under the sun, because everything is futile and a pursuit of wind. (Kohelles 2/17)
Metzudos Dovid explains the simple meaning of this possuk is that Shlomo HaMelech laments mankind for two reasons. Firstly, because the vast majority of their actions don't make any change in the world and are therefore really worthless and, secondly, nobody ever accomplishes what they set out to do.
Zera Shimshon, however, explains that this possuk alludes to something very specific.
In Sefer Shoftim (1/24-26) it is related that in the early days right after Bnei Yisroel entered Eretz Yisroel and they were involved with conquering the land, a certain person showed the scouts of Bais Yosef the entrance of a city in order that Bnei Yisroel would be able to conquer it. It is written after that, that he later built a city called Luz.
The Gemara describes a very special characteristic of this city that this Kenanni built; It is the same city Luz ... where, Senncherev did not disarrange and exile its inhabitants; Nebuchadnezzar, did not destroy it; and even the Malach Hamovess has no permission to pass through it. Rather, its Elders, when they have decided that they have reached the end of life, go outside the city wall and die. The Gemorro learns an important lesson from this with a kal v'chomer; Since this Cananni, who didn't even explicitly tell them where the city entrance was, but only motioned to them, and did not walk with them by foot, but merely indicated the correct path to them, and nevertheless caused himself to be rescued and also had the merit to provide rescue for his descendants until the end of all generations, then with regard to one who accompanies another by foot, all the more so will his reward be great.
From here it seems that the fact that nobody dies in this city was a reward that Hashem gave to the founder of this city for the great chessed that he did for Bnei Yisroel.
Zera Shimshon quotes the Zohar that explains the reason that no one dies in Luz is because in the beginning of creation Hashem created the location where Luz was ultimately built that no one will die there. He then asks that this seems to contradict our Gemorro that holds that the reason no one dies there is reward for an act of kindness while the Zohar holds that this place intrinsically protects from death.
Zera Shimshon concludes, however, that there really is no contradiction and the Gemara also holds that this place has an intrinsic ability to give life. The Gemara means only to say that in the merit and as a reward for helping Bnei Yisroel Hashem led this man to this special place.
What is so unique about Luz that people live there forever?
Zera Shimshon explains in light of the halacha that not every transaction made on condition, must the condition be fulfilled for the transaction to be valid. If the action was said or written before the condition was stipulated then even if the condition was not fulfilled the transaction is valid. For instance, in a case where a person rents an apartment to someone on the condition that the tenant will visit the owner's elderly mother every day. If he said, "If you visit my mother every day I'll rent you my apartment" then if the tenant does not visit the elderly lady every day the owner can evict the tenant since he didn't keep his part of the bargain. (tehnye kodem l'maaseh) However, if he said, "I'll rent you my apartment if you visit my mother every day" then even if the tenant does not visit his mother he can stay in the apartment because "the action was stated before the condition" (maaseh kodem l'tehnye).
According to this, explains Zera Shimshon, Hashem grants long life to everyone on the condition that the fully keep the mitzvahs and this condition was given before the action, 'in order that you will live long and your children will live long in the Land that Hashem swore to give to your forefathers...". However, the place of Luz was given before the condition of keeping the mitzvos, therefore they will have long life even if they don't keep the condition since "the action was stated before the condition".
Concludes Zera Shimshon, this is what the possuk is alluding to: Shlomo HaMelech said that I despise life, i.e., the type of life that was promised to the inhabitants of Luz that is not dependent on their conduct, because there is no purpose in what they do because whether they go in Hashem's way or not they are promised life and therefore all that they do "is futile and a pursuit of wind"
