Journey does not demand cooperation, as it can be completed solo, but the cooperative jump mechanic is central to understanding this game. I don't intend to write an entire column about jumping in Journey, so I refer you Jason Killingworth's brilliant analysis of Journey's jump mechanic, which touches upon the spiritual and the religious. For our purposes, suffice it to say that singing long notes in Journey charges your partner's ability to commit a mid-air jump, and two people singing long notes in unison while staying close to one another can effectively fly.
The desert wanderer I controlled in Journey was dressed in a simple hood and a brown robe, but the fellow wanderer I encountered was dressed in a robe with an elaborate design, such that it looked more like a skirt to me. Perhaps that's why I eventually came to think of my fellow wanderer and partner as a she, but I think placing a gender on her had much more to do with how I related to her throughout the game.
I knew there was something special about Journey when I decided, after beginning the adventure, that I could not put the controller down because it would mean abandoning my partner, and in the moments where I thought I had lost her in the desert, I panicked. I had a responsibility to stay with and help her. I wasn't overtly thinking about Journey as being about romance until the first time she and I truly flew and it reminded me of lovemaking. That has to be the strangest realization I've experienced in all my decades playing videogames, but it was undeniable. Flying with her was excitement meshed with caution meshed with alternately leading and following and becoming a single entity. We sang to each other in long tones, making each other lighter than air, soaring through canyons and under bridges of rock, and from that moment forward I would not abandon her, even at the expense of not exploring areas I wanted to explore. We was more important than I.
When I made mistakes I felt terrible because I'd potentially hurt us in the process. There's a portion of the game where you ascend a snowy path at the base of the mountain. You huddle in the shelter of stone tablets from the wind, and when ice starts caking on your body, which slows you down, singing warms you back up. Two partners singing together can warm each other faster. Sticking to the same path and staying together becomes increasingly important in this part of the game.