Jer 17:9-10: The heart is deceitful above all things, and desperately sick; who can understand it? The LORD search the heart, I try the reins, even to give every man according to his ways, and according to the fruit of his doings

Our hearts were never designed to be followed, but to be led. Our hearts were never designed to be gods in whom we believe; they were designed to believe in God. -- Jon Bloom

For where your treasure is, there will your heart be also. -Luke 12:34

The truth is, we can't follow our hearts fully. It's dangerous. Any definition of success that starts with, revolves around, and ends with ourselves is doomed to disappoint. In the ultimate sense, true success begins by humbly aligning oneself with God's definition of it. --Alex Crain

In the book of Job it is written of God: “He gives them security, and they rely on it;

Yet His eyes are on their ways.” Job 24:23  “Does He not see my ways, and count all my steps?” Job 31:4 “For His eyes are on the ways of man, and He sees all his steps. There is no darkness nor shadow of death where the workers of iniquity may hide themselves.” Job 34:21-22  Notice the words of the psalmist David about God: “The Lord looks from heaven; He sees all the sons of men. From the place of His dwelling He looks on all the inhabitants of the earth; He fashions their hearts individually;

He considers all their works.” Ps 33:13-15 Jeremiah wrote,  “Can anyone hide himself in secret places, so I shall not see him?” says the Lord; Do I not fill heaven and earth?” says the Lord.’” Jer 23:24 As Job also said, “In whose hands is the soul of every living thing and the breath of all mankind” Job 12:10  Solomon wrote, “The eyes of the Lord are in every place, keeping watch on the evil and the good.” Prov 15:3 In other words, we can run but we can’t hide from our Maker.

So, what is God wanting from us that makes him watch us continually? God wants to know if we are evil or good. Therefore, he examines our hearts, our minds, our intents, and our actions. As Jeremiah wrote, “I, the Lord, search the heart, I test the mind, even to give every man according to his ways, according to the fruit of his doings.” Jer 17:10 God will judge our secrets one day by Jesus Christ according to the apostle’s gospel. Rom 2:16 We have to remember this, that God possess the greatest prize of all, and that being eternal life. Tit 1:2 Not only eternal life does God possess, but also heaven itself. If we are ever going to get in there, we will have to be scrutinized head to toe, inside and out, heart and soul.

SOURCE: Biblical Proof:  https://biblicalproof.wordpress.com/2014/07/03/we-are-not-alone/

Who can understand our heart? Indeed, the one who knows what to search for. On this verse, I like the KJV: “I try the reins, even to give every man according to his ways, and according to the fruit of his doings.” It suggests to me that the Lord is checking to see if we will yield to his direction…..and by that He will see what is really in the heart. And then weighs not just what we do, but whether what we do bears fruit. What we do, is yield to Him. In this verse God says that he “search[es]” the heart. If the heart is always so deceitful and untrustworthy, why does the LORD even bother to search it? “Search” implies that there is more to it than just a casual dismissal based on the belief that all hearts are deceitful and untrustworthy. After “search[ing]” the heart God makes a decision about what type of action by Him is merited. This very strongly implies that some hearts are seeking wickedness and some hearts are seeking truth.......He will weigh the heart.