Why Kaidan? [Not meant to come off rude or mean he's a cutie patootie]
It's funny, I first played ME1 well over a decade ago while waiting for a new DA game. I romanced Kaidan at the time, and never played any of the later games so sort of remained on the outskirts of the fandom. Over the years, I became convinced that romancing Kaidan was the shallow choice, the safe 'Oh, you don't want to romance aliens' choice.
I came back to ME in December, and really only set out with the intention of romancing someone different. I didn't make it halfway through ME1 before going back to Kaidan and knowing I had been right all along.
The thing that struck me was how much depth his character had in such vast array of facets. His story is of disability, of chronic illness, of trauma that leaves quiet tendrils behind, of compassion born of regret, of rigorous self-reflection that still bears uncertainty and doubt, of independence and capability that learns to co-exist with new community. Every aspect of his character was immensely captivating because he was written as a person, not just a squadmate. He is full to the brim of little quirks that feel so lived in: using the metaphor of radio signals when discussing the signs he picks up from Shepard, the debriefs that fall into philosophical pondering of their place in the universe, the small observations of the things you have to slow down to really notice, the ceaseless wonder of not just the galaxy but of those who inhabit it alongside him, the sudden lines he comes out with that read like poetry--but he's only ever written limericks, sometimes.
There's an endless plethora of reasons of personality--the natural instinct to provide guidance, the kindness and compassion, the whimsy and humour--but really what draws me to him is simply that he is inescapably compelling and possesses deep enough characterisation that it is fruitful to be compelled into knowing him.