Would you knowingly and willingly like to support someone who in turn supports your enemy working against your interests? I won't. Cause he is not just someone you don't like but he is your enemy.
The enemy has nuke in his hands. Nuke in the hands of one mad fellow is a dangerous situation. You have to be a developed nation and a democratic nation to handle nukes. We are developing, so to some extent its ok for the US. But Iran is not democratic.
I think US do not like to see what looks to be a win-win situation today to be turned into a lose-win situation for them tomorrow. They are taking every steps to safeguard their interests.
We don't have to make foolish decision trusting US or some one. We need not and SHOULD NOT compromise our sovereignity and our security. Those are legitimate concerns. But I think we are not open enough and our folks are trying to fool US and in turn would get fooled finally. Just my guess.