You seem to be projecting. You are the one with no information, other than made-up speculation, about Grant's motive. Even breaking his line is not that obvious, as Grant offered zero reason for attacking at Cold Harbor in his memoirs.
It's pretty obvious, but we have to keep going over the same old ground, because people keep responding to me, offering excuses for Grant, where Grant himself felt the need to make none. He simply regretted Cold Harbor, and made no explanation of his reason for it, although he offered a paragraph for his explanation of the assault at Vicksburg.
It was pure regret of a presumably rash mistake.