From Putin's perspective, it was probably, "If not now, then when?"
Putin met with Xi during the Olympics and got Xi's endorsement of the invasion. Between Russia and China, they probably have enough secret videos and audios of Hunter Biden's deals with their countries to blackmail Joe Biden and ensure that the US wouldn't take military action. Plus, it's unlikely that this administration would do much more than a stern rebuke anyway. An open water port in the Baltic Sea would give shorter trading lines between Russia and China. For that matter, it would provide more shipping lines to the EU.
I think it's clear to Putin that the US would not respond militarily. Biden almost said so in his speeches. Without the US, NATO isn't going to do anything but sanctions and, as others have pointed out, they need Russia's fossil fuels so the sanctions are likely to have loopholes.
Plus, if the Republicans retake the House of Representatives in November... no guaranteed of that, of course... sanctions could be worse if Putin waited.
Putin is former KGB. He's a killer. He apparently wants his legacy to be reuniting the old Soviet Union.
So if not now, when?
Still...
"No plan survives first contact with the enemy."