When you've been hurt a lot, you tend to build walls and harden your heart. When you can't trust anyone, you build walls. It's a defense mechanism. Sometimes we just want to be safe. We get so tired of giving our heart away to people and then have it stepped on and crushed afterwards. It's a terrible feeling, and so then you say "I won't do that next time." So you learn to stop trusting people. You start to harden your heart so you are not hurt like that again.
But not trusting anyone is a VERY lonely feeling. I get it, sometimes it's nice to be alone. But to feel alone all the time doesn't feel good at all. We're social creatures, we weren't meant to be alone. You want to reach out. You want someone you can really trust. You want someone who really cares about you. Maybe, you just want someone to talk to, or someone who will listen to you.
We weren't made to have hardened hearts. God gives us a heart of flesh, not stone (Ezekiel 36:26). Are we going to get hurt sometimes? Of course, there's no way to avoid getting hurt. But having a tender heart also means you care for others. You have compassion for others. You have empathy for others.
I'll admit, there have been times where I actually asked God to harden my heart, weird right? Like He's really going to do that! I don't ask for that anymore though. I realize that with love comes pain. You can't love others, but not feel any pain at the same time. If I want to help other people, I have to have a tender heart. If I want to show people the love of God, I have to have a tender heart. Jesus had a tender heart while he walked on the earth. If Jesus had a tender heart, shouldn't we as followers of Christ have tender hearts as well? You know he had a tender heart because he had compassion for people. He loved them more than anyone ever could love them. I mean think about it, he died for all of us because he loved us. His heart melted like wax (Psalm 22:14). If this is the heart of Jesus, then we as followers of Christ need to have a heart like that as well.