The whole point of trying to engage Iran is the same as with Cuba. It has been a LONG time both were in isolation. Lets extend a hand and see if they want to rejoin and work with the global community. At least Iran has something resembling elections unlike the KSA, and the most recent elections brought in more reformers as the young people see the reformist president was able to make a deal and actually may be able to improve their lives. People know how their lives were before, if we can show them that through cooperation (both economic & diplomatic) things improve relations and their tone will most likely improve. (aside from the hardliners but come on look at the far right wing in America, those people always exist and will never change their views)
Look at the situation where that US patrol boat drifted into Iranian waters, that could have been a mess a few years ago. It was handled quickly and with no harm diplomatically because both sides have had communication and have built some trust.