How nodes agree to consensus rules? Is it possible for someone to change the consensus rules for his own software and then remain in the previous network? If not, then why?
If you alter the consensus rules in the software running your node, there are two options:
You apply rules less restrict than before, accepting some transactions that were previously invalid: every block you mine will be displaced by blocks from longer chains mined by the miners that didn't change anything and have more hashpower than you. Because they'll never accept your blocks but you're still accepting theirs.
You apply rules more restrict than before, rejecting some transactions that were previously valid: you are on your own, and will have to mine every single block. Depending on your current hashpower, it could take weeks until the next one. Unless you also change the rules concerning minimum difficulty. But the blocks you mine still will be accepted only by your node.