With all due respect, you can only make assumptions like this by ignoring most of what Grant actually says in that report.
You appear to be quoting from the Report of Lieut. Gen. U.S. Grant, U.S. Army, Commanding Armies of the United States, Of Operations March, 1864 - May, 1865 which was sent on July 22, 1865 to E. M. STANTON, Secretary of War. Here are the first five paragraphs, with the part you are quoting in
light blue text. Looks like it is actually the 4th paragraph if this is the document and it appears to be part of a sentence instead of a complete sentence. The "first" part of the sentence is in
underlined bold text.
SIR: I have the honor to submit the following report of the operations of the armies of the United States from the date of my appointment to command the same:
From an early period in the rebellion I had been impressed with the idea that active and continuous operations of all the troops that could be brought into the field, regardless of season and weather, were necessary to a speedy termination of the war. The resources of the enemy and his numerical strength were far inferior to ours; but as an offset to this, we had a vast territory, with a population hostile to the Government, to garrison, and long lines of river and railroad communications to protect, to enable us to supply the operating armies.
The armies in the East and West acted independently and without concert, like a balky team, no two ever pulling together, enabling the enemy to use to great advantage his interior lines of communication for transporting troops from east to west, re-enforcing the army most vigorously pressed, and to furlough large numbers, during seasons of inactivity on our part, to go to their homes and do the work of producing for the support of their armies. It was a question whether our numerical strength and resources were not more than balanced by these disadvantages and the enemy's superior position.
From the first, I was firm in the conviction that no peace could be had that would be stable and conducive to the happiness of the people, both North and South, until the military power of the rebellion was entirely broken. I therefore determined, first, to use the greatest number of troops practicable against the armed force of the enemy, preventing him from using the same force at different seasons against first one and then another of our armies, and the possibility of repose for refitting and producing necessary supplies for carrying on resistance; second, to hammer continuously against the armed force of the enemy and his resources, until by mere attrition, if in no other way, there should be nothing left to him but an equal submission with the loyal section of our common country to the constitution and laws of the land. These views have been kept constantly in mind, and orders given and campaigns made to carry them out. Whether they might have been better in conception and execution is for the people, who mourn the loss of friends fallen and who have to pay the pecuniary cost, to say. All I can say is, that what I have done has been done conscientiously, to the best of my ability, and in what I conceived to be for the best interests of the whole country.
None of that says anything at all about Grant losing his aggressiveness. He continued to be aggressive from the first to the last. You seem to want to see only the tiniest portion of what was said and done so you can fit it into your conception. When he reached Petersburg, Grant realized his Virginia troops were exhausted for the moment. They needed rest and refit before the campaign could be continued with the AoP.
What you miss is that Grant does not command the AoP. Grant commands the combined Army of the United States. Grant commands Sherman and Banks and many others. Grant commands ===ALL=== of the others. When he decides to change to ***greater*** emphasis on the offensive on other fronts, Grant is still being aggressive, still pushing the enemy -- and he pursues a very active siege in Virginia to keep the pressure on Lee and assist his other commanders.