At the beginning of Suleiman's reign the Safavids were still reeling from the defeats that had been inflicted upon them by his father Selim I. They quite desperately needed peace with the Ottomans, and such gestures of friendliness were an easy way to try to score diplomatic points with Istanbul, so to speak. Suleiman did ease up on the Safavids at first, for a variety of reasons, but particularly because the troops were tired of eastern campaigns, and he wanted to use campaigns in the west to buttress his political position vis-a-vis the viziers he inherited from his father. So during the intial period of Suleiman's reign, before the resumption of war with the Two Iraqs Campaign in 1534, there was peace and somewhat better relations between the Ottomans and Safavids.

Ultimately though, the Ottoman-Safavid relationship was irreconcilable because of the ideological, religious, and imperial rivalry between them. Real and lasting peace would not come until 1639. An alliance at this time would be implausible - what third party would they ally against?

On Suleiman's reign and the Ottoman-Safavid rivalry you can look to Kaya Şahin's Empire and Power in the Reign of Süleyman: Narrating the Sixteenth-Century Ottoman World. Cambridge: Cambridge University Press, 2013.