Thistle's life experience with Derghal is...
surprisingly normal, if not lively. He must really love him so much that he's been trying to make him immortal.
He's a foil to Marcille, who in the end accepts that races' life span varies and death is normal and to be accepted. He himself felt serene to know YaadDerghal is fine so he felt peaceful in the end.
In retrospective, if Derghal accepts death as one of the package of life, Thistle might've not gone awry.