Here it is we see that God does not care about outward displays of repentence (rending your garments) as much as the inward display (rending our hearts). There is something about laying ourselves open and vulnerable before Him that not only pleases Him, but touches us, changes us, improves us. When we lay our hearts- bare, bruised, and bleeding- before Him, He is "softened". It's like when a child comes hurting to their father... they've done something wrong, but they are torn by it. They are sorrowful and repentant. It touches the very heart of the father as they look at this child, coming to them in humility and admitting they need help.
"Yet even now," declares the LORD, "return to me with all your heart, with
fasting, with weeping, and with mourning; and rend your hearts and not your
garments." Return to the LORD your God, for he is gracious and merciful, slow to anger, and abounding in steadfast love; and he relents over disaster. Who
knows whether he will not turn and relent, and leave a blessing behind him, a
grain offering and a drink offering for the LORD your God?
Joel 2: 12-14
There is something so powerful about laying ourselves down completely before God. We then allow Him to take control and do a work in us. In the beginning of that passage He says "Yet even now". Yet even in our wrongdoing... even when we are reaping the consequences of what we've sown, He will still receive us, for He is "gracious and merciful, slow to anger, and abounding in steadfast love". This isn't to say that we can then do whatever we please, so long as we come at the end of the day and lay ourselves before Him. Rather, truly laying ourselves before Him- open, vulnerable, and in all humility- will result in a true transformation of the heart. Don't think you can fake humility with God, He is God. The same God that read the hearts of the Pharisees while He was manifest here on earth. He can read your heart, too.
Now for some (if you are anything like me) humility is hard. It's a struggle, not only being humble, but knowing if you are truly humble, or whether you are putting on a disguise. And so for me, because it is so difficult, I have to rely on God to help me open my eyes in this area. I might pray "God, please humble my heart. Show me the humility of Christ and help me apply that in my own life. If there is any pride in me, please remove it now." And generally once I've prayed I can either sigh in gratefulness at God's work, or I can uneasily know that I will need to take some steps to remove the pride still there. But once the pride is gone, you can then easily move on. You can rely more fully on God. You can come to God and say "God I don't deserve your mercy and grace, but that You would give it to me is more than I can express. That You would lay down all of You for my soul, is too beautiful, holy, and loving than I can fathom." When you accept the pardon God is offering here you can return to the original design of man and God walking together in a relationship with one another.
Another thing I'd like to point out, is that God isn't asking here. He's commanding. He's declaring. He's decreeing. The consequence of not obeying this command is simple- Hell, eternal condemnation, the bondage to sin forever. Because while God is a God of grace and mercy to those that humble themselves before Him and accept the pardon, He is also a God of justice and righteousness.
So yes, God loves grace and mercy, but for those who don't obey, He, being a righteous and just God must punish them. It doesn't make Him mean, cruel, or indifferent. It exalts Him and shows His holiness. For in all things God can be glorified. If not now, then later. In the end all things shall point to God.
But the LORD of hosts is exalted in justice,and the Holy God shows himself holy
Isaiah 5: 16
Return to God... He cares. He cares enough to give up the one thing that was hardest to give. His perfect and holy Son. He loved each person that much. And as my favorite line in the movie Flywheel goes "Now you go get right with God before I get you right with Him."