Because people today rely on things to make them happy more than they rely on God. Seeing how fast people get over things that made them happy there is always going to be emptiness inside of them because nothing satisfies them for very long. Today people are chasing sonething that ultimately doesnt bring them happiness even though it seems like it would. People today are chasing a lie. Theirs nothing more depressing than that. Only in Christ is there true happiness. He gives a happiness that lasts forever as long as you are obedient to God. When you have Gods Spirit in you there will always be happiness joy and peace inside of you and materialistic things wont mean a thing to you. Why's everyone depressed today. Well the truth is because most people are living in sin. Sin seems fun but it leaves you broken and depressed because you're constantly chasing and doing something that's actually killing you slowly. Sin kills you physically mentality and spirituality.
7 Be not deceived; God is not mocked: for whatsoever a man soweth, that shall he also reap.
8 For he that soweth to his flesh shall of the flesh reap corruption; but he that soweth to the Spirit shall of the Spirit reap life everlasting.
19 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness,
20 Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies,
21 Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.