Pretty simple, in my view. Trump is a megalomaniac who has on one occasion claimed to have been in Moscow and met Putin, then claimed to have been met him on the TV show 60 Minutes (they were on separate sets continents apart), then when the heat was on claimed not to know him. The one constant that runs through this so-called relationship is that Trump is easily flattered. Putin, the old KGB puppet master, would occasionally feed his ego with a few compliments.
Trump is a weak little man who's ill-prepared for even local public office. He very likely became an unwitting stooge for the Russian Federation. Putin may be running his country into the ground economically and is most certainly a threat to regional stability, but he can still eat vain, deluded people like Trump alive.