In these verses, the author's tone becomes a lot more corrective as he issues a number of stern warnings. There is no doubt this is motivated by love, as it's the same author who earlier in the chapter reminded us that God loves those He corrects.
In light of the glorious and incredible truth we are seeing in the book of Hebrews, that God has made available to us in Christ - it makes sense that there should be a righteous indignation towards any who would take this for granted, neglect it, or refuse it with contempt. How can we neglect so great a salvation? How can we think our way is better? How can we think we can prosper, by throwing back in God's face all He has made available to us? After all, our God is a "consuming fire", which understood correctly should not drive us from Him, but towards Him. This is the focus of our passage here in chapter 12.