The Chidushei HaRim zt'l teaches that we read Koheles on Succos to remind us that joy isn't the product of hevel havalim, or frivolity. True simchah is when it is connected to Hashem, to His Torah, and His mitzvos.
Luxuries
It states וה' ברך את אברהם בכל "Hashem blessed Avraham with everything." The Midrash says that בכל "everything," refers to the mitzvah of succah. This means Hashem blessed Avraham with the mitzvah of succah, which is בכל "everything." How is this mitzvah everything?
The Chozeh of Lublin (Avnei Zikaron 662) zt'l explains that the succah reminds us of the vainness of luxuries. During Succos, we live in a temporary hut for seven days, and all is fine and well. We discover that luxuries are not as essential as we thought they were. And when we know this, we have everything. Even if we have fewer physical possessions, we can be satisfied with our lot.
When we think we can't live without luxuries, we are never satisfied. Whatever we have, we need more. It is never enough. We need a renovated kitchen and a new car. When we make a simchah, we need to copy the wealthy, or it won't be good enough. We think that vacations must cost a lot of money, or they aren't enjoyable. This results in financial debt, and then with hard work to cover the expenses, which means less time for Torah and mitzvos, for things that truly matter. We are always unhappy because we feel that we don't have enough, yet. Succah is בכל, "everything." We attain awareness that we can have little, and yet have a lot in life.
It states (Bereishis 25:8(: ויגוע וימת אברהם בשיבה טובה זקן ושבע "Avraham passed away at an old age, old and satisfied." The Siforno translates as "He was satisfied with everything he desired to see and to do in his lifetime." This is a great and rare level. Rabbeinu b'Chayei writes:
"This pasuk reveals Avraham's good middos. He left this world feeling fulfilled. He didn't need more. This is unlike most people about whom it states (Koheles 5:9(אוהב כסף לא ישבע כסף,'One who desires money will never have enough money.' And Chazal (Koheles Rabbah 1:32) say, 'When a person leaves the world, he hasn't attained half of what he wanted. If he has one hundred, he wants two hundred. If he has two hundred, he wants four hundred.'" But Avraham was satisfied and happy with his lot, and that is a wonderful and unique level. He attained this level because he was blessed בכל, with everything, which means he was satisfied with his portion.
Rebbe Yonason Eibshitz (Yaaras Dvash 6:1) writes, "For the person who fears Hashem, this idea doesn't only apply to Succos. Throughout the year, he will remember that life is temporary and that he's a guest in this world. And, therefore, he shouldn't build expensive houses decorated with gems and diamonds for his pleasure. This will cause him to desire more money and to sin, and he will become a baal gaavah and forget Hashem Who created him. Remember Yonadav ben Rechav's precious, ethical will (see Yirmiyahu 35:6). He instructed his children not to live in houses, only in tents, one day here and the next day somewhere else. Our forefathers, Avraham, Yitzchak, and Yaakov did that. They dwelt in tents, and they had long lives. 25 They didn't have to worry about fires burning down their castles. They didn't live with the fear of enemies, hunger, or plague. They could quickly undo the ropes of their tents and travel elsewhere.
"When Bilaam saw the Jewish nation מה טובו אהליך יעקב ,dwelling in tents, he said 'How good are your tents.' 'Tents', in this pasuk, is intentional because we are a nation that dwells in tents... and that is good for us in this world and the next. But when we build houses of lime and stone, the stones will shout out all the sins of theft, pride, temptation, and desire. This is the lesson of the mitzvah of succah..."
25. Reb Yonason Eibshitz also notes that the Mishkan, made from יריעות skins, and woven cloth, wasn't a permanent structure. Yet, it lasted 480 years. The Beis HaMikdash that Shlomo HaMelech built was made of stone and gold, and it stood for less time – 410 years. The second Beis HaMikdash lasted 420 years, and during the 420 years, the Greeks ruled part of the time, and the Beis HaMikdash was destroyed several times. So, we see that a דירת ארעי a temporary dwelling, can last longer than permanent structures. Similarly, he writes, the avos owned few luxuries, and therefore, they lived long.