Totally nice guys, all on the up and up.

Has nothing to do with "nice." It's all about incentives.

A regime like Iran wants nuclear weapons because they confer a lot of power - and especially a huge defensive deterrent. If you have a nuke, it's probable that no one will ever invade you to overthrow your regime. If Iran signals that in the event of invasion the last act of their government would be to vaporize Israel, they'd be relatively safe from military force against them from either Israel or the West. They're vastly better off having a nuke than not.

However....Israel and the U.S. know this. So if the current regime were to try to actually develop a nuke, that might precipitate Israel or the U.S. overthrowing the regime pre-emptively. We don't want chaos in Iran...but we don't want them to have a nuke, either. If it looks like they're getting close in a way that we can't stop by physically attacking the nuclear program itself, then either the U.S. or Israel or both would invade to overthrow the government and stop them.

So having a nuke protects you from regime change, but pursuing a nuke can precipitate regime change. Conflicting incentives. Iran's response has been a rational one, from the regime's point of view: get far enough along so that you can move to a nuke quickly in extremis, but not so far as to trigger an invasion. The reason they don't go from 60% to 100% right now is because they don't want to get attacked.

If you make them feel like their regime is safe from direct military action from the West, then they don't have a massive incentive to rush to a nuke. Because the rush is very dangerous for them, so it's not worth the risk if they are otherwise in a stable position.

If they end up getting attacked anyway, then their incentives change. If it looks like the West is going to try to overthrow their government anyway, they now have 100% incentive to just try to race to a nuke. They only need a delivery mechanism capable of putting it somewhere between Jerusalem and Tel Aviv to guarantee their safety.

So even though they've always wanted a nuke, they faced powerful incentives not to get close enough to it for the West and Israel to decide to just take out the regime. Attacking them shifts those incentives....