We are warned not to allow ourselves to become hardened, because if we look at the whole concept of hardening in its biblical perspective, we see that something happens to us through repeated sins. Our consciences become seared. The more we commit a particular sin, the less remorse we feel from it. Our hearts are recalcitrant through repeated disobedience.
When God hardens the heart, all He does is step away and stop striving with us. For example, the first time I commit a particular sin, my conscience bothers me. In His grace, God is convicting me of that evil. God is intruding into my life, trying to persuade me to stop this wickedness. If He wants to harden me, all He has to do is to stop rebuking me, stop nudging me, and just give me enough rope to hang myself.
We see in Scripture that when God hardens hearts, He does not force people to sin; rather, He gives them their freedom to exercise the evil of their own desires (James 1:13–15).