Something that really stuck with me while I was playing Mass Effect 2, and that I feel was so important to understanding the relationship between Shepard and Garrus, whether you romance him or not, is that Garrus is your only companion from ME1 who never moved on.
You died. The crew watched you die. Assumedly they grieved you, there was a funeral service, there were tears and all five stages of grief. Things were rough, but eventually life took on a shade of normalcy again.
Kaiden/Ashley got reassigned and continued their work for the alliance. Liara fights tooth and nail to get your body to Cerberus, but then it’s out of her hands and she becomes an information broker. Tali goes back to the Floatilla, like she was always supposed to. Wrex becomes a clan leader. And yes, they’re all fundamentally changed by their time on the Normandy, but no one seems as stuck as Garrus Vakarian.
Because he doesn’t go back to the Citadel. He doesn’t join the Turian Hierarchy. He ships himself off to Omega, one of the most dangerous places he could be, and makes a life out of pissing off the most dangerous people there. It’s kind of a suicide mission, especially after what happened with Sidonis. He kind of figures he’s going to die doing this, but what else can he do?
There is no normalcy for Garrus in a world without Shepard, and I think that’s why he’s always immediately ready to come back to the Normandy. The Normandy, and Shepard, are home for Garrus and that’s why I think romancing him feels like the natural conclusion for so many femme Sheps, because Shepard feels that way too.
Of all the companions, no one needs Shepard to live more than Garrus, and no one needs Garrus more than Shepard. They’re perfectly matched, as lovers, as friends, as soldiers, as whatever they are in your universe and when you’re that perfectly matched with someone, you never quite feel whole without them.
No Shepard without Vakarian.