As I have always said, unnatural alliances have their own trip wires and don't last in the the long term. Indian relationship with Iran is one of those " UNNATURAL RELATIONSHIPS ". Pakistan on the other hand is IRAN's NATURAL ally with a common culture, religion and geography ( common borders ).
This is why as India goes into convulsions relative to its relations with Iran because of pressures from Israel and the USA, while Pakistan has no such issues. When the US applies pressure on Pakistan with regards to Iran, Pakistan tells the US to go jump in a lake and also Iran and Pakistan see eye to eye on Israeli issue so no problem there.
Natural allies think alike and that makes the relationship effortless. Unnatural allies make alliances because of expedience which is like building castles with sand ( i.e. does not last ).
India - Iran relationship is such unnatural alliance and hence the convulsions.