If we look at Zack and Cloud, especially at the way they're portrayed or rather headcanonned in the community, we'll see that comfort plays a significant role in their relationship. Let me digress here and post a link to a
curiously misplaced discussion about whether the two really were best friends in the Compilation. Licorice and Makoeyes give perfectly reasonable arguments why they think the boys were or were not close by the end of Crisis Core, and the truth is probably somewhere in-between. If SE wants us to believe they were best friends, they definitely failed to portray it properly throughout the game, because it lacks episodes showing Cloud and Zack opening up more to each other or simply spending more time together. Basically, the case of Noctis and Luna: the player is told they're in love but isn't given enough screentime to invest in the pair emotionally and therefore is left in disbelief. So the community picked up the pieces and reconstructed the picture giving ZC all the time in the world. So we have Cloud who is a walking mass of complexes, socially challenged, and in need of someone who would accept him the way he is. And we have Zack who is willing to befriend everybody and envelop them in his bursting energy. Extremes meet, don't they? A perfect opposing dynamic where one is charging and comforting the other, whereas the other needs to be charged and comforted.