Why is it necessary for Russia to hit U infrastructure in retaliation? How about because it's there...
I think the reason we were hitting substations and transformers during the Winter of '22-'23 is because:
1) We didn't want to lose points with the Ukrainian population by plunging them into cold and darkness (this season's attacks only came after the winter had ended, and the problems they'll have this coming winter will be squarely blamed on their own government instead of us).
2) We hoped to capture those places intact, like we did with the Zaporozhye Nuclear Power Plant (the largest in Europe), and the Uglegorsk Thermal Power Plant (also one of the largest in Europe).
3) The main focus was on disabling Ukraine's rolling stock (most Ukrainian railways are electrified). By disrupting rail traffic, the hope was that it would create difficulties in supplying the front in the East.
In the end, the trouble begins with the hope of winning Ukrainian "hearts and minds". As it turned out, it didn't matter what Ukrainians thought about us, so long as their criminal government remained in place.
At the same time, there have been a great many people on the Russian side that have been advocating from Day 1 that we should:
1) Strike the Presidential Administration, Ministry of Defense, Main Defense Intelligence directorate, and the headquarters of the Security Service of Ukraine.
2) Disable all power plants, with the exception of nuclear ones (for fear of causing radioactive fallout).
3) Destroy bridges across the Dnieper.