Man’s preoccupation with self

Man’s basic problem is preoccupation with self. He is innately beset with narcissism, a condition named after the Greek mythological character Narcissus, who spent his life admiring his reflection in a pool of water. In the final analysis, every sin results from preoccupation with self. We sin because we are totally selfish, totally devoted to ourselves, rather than to God and to others.
~John MacArthur

If you have lived entirely for self there must be a reckoning with you

You will live somewhere or other after your present career is closed. Does it not stand to reason that if you have lived entirely for self there must be a reckoning with you? Somebody made you! God made you! If you keep a horse or a cow you expect some service of it, and, if God made you, he must expect you to render him some service. But you have rendered him none. Though he has winked at your disobedience in this life, do you think he will always wink at it? Well, if you do think so, you are grossly mistaken: for, as the Lord liveth, there is a day of judgment coming, when the Lord Jesus Christ shall descend from heaven with a shout, and all the dead shall rise out of their graves, and all the living shall appear before his great white throne.

You will as certainly be there as you are here. And when you are there, you will discover that every secret thought of yours has been written down against you, and will be read out and published before mankind, and there and then for every idle word you have spoken you will be brought into judgment. Can you think of this as possible, even though you may not admit that it is certain, and can you yet remain callous, indifferent, unconcerned? Is there not a something in your heart that says, “If this be so, it is terrible — it is terrible for me! What must I do to be saved? “I am bound to answer you (and cheerfully do I answer you), “Believe in the Lord Jesus Christ, and thou shalt be saved.”

~ Charles Haddon Spurgeon

There is a double nature in all believers

There is a double nature in all believers. Converted, renewed, sanctified as they are, they still carry about with them a mass of indwelling corruption, a body of sin. Paul speaks of this when he says, “I find a law, that, when I would do good, evil is present with me. For I delight in the law of God after the inward man. But I see another law in my members, warring against the law of my mind” (Romans 7:21-23). The experience of all true Christians in every age confirms this. They find within, two contrary principles, and a continual strife between the two. To these two principles our Lord alludes when He addresses His half-awakened disciples. He calls the one flesh and the other spirit. He says, “the spirit is willing, but the flesh is weak.”
~ J.C. Ryle