Because although the pair have spent much of the past few weeks flirting over Instagram, no one saw an engagement coming so soon.
However, we really should have been paying more attention to these flirtatious messages, because Pete may have hinted at their engagement on Saturday.
So here's what went down. Ariana shared this photo of herself with Pete on Saturday, alongside a cute caption.
And Pete then commented with this.
At the time, I was just like: "Aww, cute." But now I'm wondering whether this caption was actually in reference to their engagement.
Because it does sound mightily like the sort of thing you'd say to the person who had just accepted your proposal.
What's more, the photo was posted on Saturday night – the same evening that Ari and Pete were reportedly telling all their friends about the engagement.
So what at first appeared to just be a cute Instagram interaction may have actually been Pete expressing his joy at their engagement.