Why are people sad? It is generally due to jealousy. They would be happy with their lot if it weren't that others have more than they do.
Chazal (Avos 4:21) tell us, את מוצאין ...הקנאה העולם מן האדם, "Jealousy...drives man out of the world."
Reishis Chachmah writes, "Someone with kinah is 'robbing' himself because he is always sad. Everyone can find sweetness in life, except for someone who suffers from jealousy. He cannot enjoy his food if someone else has something good."
The Rosh (Orchos Chaim) writes, "Jealousy is a disease that does not have a cure."
The Mesilas Yesharim (ch.11) writes, "It is foolish to be jealous because what does one gain from it? The only one who loses is him. Some people are so foolish that when they see their fellow man has something good, they become very distressed and upset. Because of their sorrow, they don't enjoy anything they have... Others don't become so upset, but they also have some remorse. They aren't calm when they see someone reach a level above them (unless this person is their very close friend). This describes most people. They aren't baalei kinah (constantly filled with jealousy), but they aren't pure from it either..."
It states (Mishlei 14:30) קנאה עצמות ורקב, jealousy causes one's bones to rot. If we could free ourselves from this illness, we would lead much happier and satisfying lives.
This subject also applies to spiritual pursuits. For example, many people are jealous of those who know a lot of Torah, enjoy the spirituality of Shabbos, daven well, are very involved in chesed, and the list goes on. Sometimes, jealousy is beneficial. As Chazal say, חכמה תרבה סופרים קנאת, jealousy increases wisdom. But often, jealousy causes people to belittle themselves and to feel bitter and unhappy with the good that they do succeed in doing.
We can overcome jealousy with the emunah that Hashem gives everyone precisely what he needs. So, if someone has a larger house or a nicer car than you, it's because Hashem determined that that person needs it (for his avodas Hashem), and you don't. Therefore, there is no reason to be jealous of others. As a tzaddik said, "If I don't have something, that means I don't need it. Because if I did need it, I would have it."
In regard to spirituality, remember that Hashem wants a different service from each person. Everyone creates a unique nachas ruach for Hashem. So why be jealous of others? They aren't doing what you can do.
The following mashal brings out this point.
In honor of their father's eightieth birthday, his three sons decided to throw a party. They wanted to lift the spirits of their father, an aging widower.
His oldest son was very wealthy. He bought his father a three-story house. At the party, he presented his father with the contract, together with beautiful pictures of the house.
His second son was a middle-class earner. He bought his father a used car, which was an upgrade from the jalopy their father drove at the time. He gave his father the car keys and showed him pictures of the vehicle.
The youngest son was poor. He couldn't afford a gift. So, he wrote a letter and read it out loud at the party.
The sons were sure their father would be happiest with the house, the car next, and the letter last.
Their father surprised them when he got up and said, "I live alone. I don't need a three-story house. The car is good, but I hardly drive. My old jalopy is good enough for me. But my youngest son gave me the best present of all. His letter expressed gratitude for all I did for him all these years, and that's the best gift of all."
Similarly, we don't know which deed is more beloved by Hashem. You might think your deeds are mediocre or trivial, and you are jealous of those who seem to do much more, but your simple deeds may be the most beloved of all.
