Mendes was remarkably open about Cabello in his Netflix documentary, In Wonder, admitting that "every song [he's] ever wrote" has been about the "Living Proof" singer. One song on his new album, "24 Hours," talks about getting serious quickly in a relationship. Mendes tells ET's Katie Krause that he and Cabello have discussed getting engaged.
"Yeah, absolutely, I think for me it's always been if I'm not willing to talk about that stuff then I kind of check out a little bit, and I don't know, I really think when you know, you know," he says of Cabello. "She's been one of my best friends since I was, like, 15 years old. I don't know, at the same time, I know we are really young so I don't want to jump, like, insanely fast, but I think when you find your person, you feel and you know that you have found your person."
And when it comes to their music, Mendes says nothing is off limits when they write songs about one another.
"No, we really don't put any rules or boundaries on what we are writing about or in our art," he says. "I think the truth about being celebrities and being in a relationship is that it's super tricky, because there are a lot of people who want to know more than what is maybe normal just to give out, they want more personal stuff. But both of us realize that it is just sometimes you say something that maybe you shouldn't have, and someone asks you a personal question and you answered it because you were off guard, and you know, you can't really do much about it. It's just kind of the nature of the beast."
But both 22-year-old Mendes and 23-year-old Cabello aren't afraid to share their feelings about one another through their songwriting.
"I think we are both equally very mushy, and I think we are both extremely mushy people," he says when asked whom of the two is more mushy when it comes to writing songs. "I think we are both super sensitive which creates a lot of sensitivity in our relationship, but we are songwriters so we are constantly searching for the best topics in life and I don't know, I am pretty mushy. I don't know what she would answer or how she would answer that. I feel like she would probably say I am much more mushy."
Aside from Mendes' holiday plans -- which he says will involve him and Cabello staying in the basement of his parents' house in Ontario, Canada, with their new puppy, Tarzan -- Mendes is of course focused on his new album, which is extremely personal to him. He wrote or co-wrote every single song on Wonder.
"The album is like a reflection of my life at the moment, you know, or my life I guess I should say rather, over the last six to seven months," he shares. "I had a big moment of anxiety and fear and I felt like I wasn't able to make music at the top of the year. And I think that was all coming from a place of fear, of people not liking the music I made, and it was kind of choking me as a songwriter -- or fear of maybe not being able to make music better than I made in the past. So, I gave myself a month to kind of think about, figure out what was at the bottom of that. And I had a lot of fear regarding my art."
Mendes says he was able to create Wonder after freeing himself from expectations.
"I found a place after a lot of work and a lot of kind of self-meditation and journaling and a bunch of different fun things that helped me where I went in and I started making an album," he recalls. "And I just went in with the thing of like, 'Hey, like, this is it. I showed up, I'm here to make music, I'm here to make art, and if it happens in the best way, it does, and if it doesn't happen, then it doesn't. And I don't really have control.' And ironically when I freed myself from that expectation, I think I started making my favorite art that I've ever made in my life. I mean ... I really feel like I created an album that feels like freedom. ... I felt very free just making it. I felt like this is music that I love to listen to, this is music I want to hear, this is music I want to make and it's gorgeous for that reason."
For more from Mendes on Wonder, watch the video below.