Iran is a theocratic Islamic state fueled by hatred of the west.
Tehran killed Americans in Lebanon, worked with Al-Qaeda in Sudan, and even had a significant roll in the 9/11 attacks according to the 9/11 Commission and overwhelming legal evidence presented in American court.
According to both the Trump and Obama administrations, Iran is the world's leading sponsor of terror. Trump recently observed that Iran gave safe-haven to Osama Bin Laden's son and continues to actively aid Al-Qaeda today.
Putin is friends with the Mullahs who run Iran, oppress their people, and spread terror through the world. On every level.
Russia has been publicly helping Iran with nuclear technology. All the way back in 1995 they began work on a nuclear plant in Bushehr. This work accelerated under Putin. In 2011 the plant was finished giving Russia the distinction of building the first nuclear "power plant" in the Middle East.
Sergei Kiriyenko, head of the Russian Federation of Atomic Energy, bragged about completing the nuclear plant despite the "adverse political situation."
Iran and Russia recently celebrated the inauguration of construction for a second nuclear "plant."
Of course, no nuclear reactor is complete without air defense. Putin has completed a deal to sell Iran sophisticated S-300 missile defense systems.
Putin's nuclear and military aid is backed by economic and diplomatic support. Putin is ramping up economic trade. And defends Iran diplomatically over sanctions due to Iran's dangerous nuclear activity, ballastic missile development, and support for terror supporting genocidal Assad.
Putin's close relations with Iran should come as no surprise given that Iran's Supreme Leader, Khamanei, was a graduate of Moscow's Patrice Lumumba's People's Friendship University.