Well here goes my headcannoning about how my shep fell in love with our favorite Turian:
Joan Shepard is a Spacer/sole survivor and a paragon with renegade moments (especially in ME2 because she's got a hefty amount of anger about the whole being dead for two years and nobody doing anything about the reaper threat during that time)
She was in a relationship with her squadmate, who died on Akuze and during ME1 she was still mourning him and the last thing she wanted was to get romantically invovled with another crewmate so she was pretty harsh with Liara and Kaidan when they expressed their interest in her.
She and Garrus became buddies in ME1, although she didn't approve of his hot-headedness and thought he might be too much of a renegade at times. She recognized they shared a thirst for justice in the world, although they have diffrent ideas of what qualifies as justice and what the limits are: Joan's philosophy being that vengence breeds more vengence where, as we all know, Garrus is more of an 'eye for an eye' kinda guy.
I think falling in love with him snuck up on her.. and then hit her like a hammer. When he got hit by that rocket she was completely distraught and it felt like loosing her lover on Akuze all over again... she realized her feelings for him were stronger than she thought and that scared the hell out of her.
I think in ME2 it's obvious that they really care about each other deeply but they turn to humor whenever the subject of their emotions rears it's head... since Garrus (ever the pessimist) is just waiting for the moment that he screws it all up and Joan is terrified of her own feelings. I think during their first night together though all the joking and akwardness falls to the wayside (like in the scene how it shifts from G's joking to his honest confession that he just want's something in his life to go right) and for those couple of hours they just let go of all the horrible crap that's going on and find solace in each other.
Whew sorry for the wall of text.. that came out longer than I thought it would..
I should probs just start up a fanfiction about these two.