Now that there's a wider acceptance of the Faith/Lindsey 'ship, it's no longer necessary to give a detailed reasoning behind why the pairing works. Not that a detailed reasoning was ever really required, because it's pretty simple: their characters are more alike than either of them are in comparison to most others on the show(s). Although we meet them at different points on their journeys, the story of Faith and the story of Lindsey are very similar.

Faith grew up in Boston, and when we first see her in the season 3 episode of Buffy: the Vampire Slayer, 'Faith, Hope and Trick', she's got a smart mouth, is good at what she does, and, by the end of the episode, has seemingly conquered the trauma in her past.

Lindsey grew up in Oklahoma, and when we first see him in the season 1 episode of Angel: the series, 'City Of', he's got a smart mouth, is good at what he does, and shows no sign of having any kind of trauma. However, in the later episode 'Blind Date', Lindsey reveals that his family was extremely poor through his childhood and several of his siblings died from illness. The only parent he makes a mention of is his father.

Faith's mother (the only parent she mentions) was an alcoholic, and has died by the time the girl arrives in Sunnydale. She used to call Faith her firecracker, but was too busy 'enjoying the drinking and falling down parts of life' to care for her daughter.

By the time Angel moves to LA and encounters Wolfram and Hart, Lindsey has conquered the poverty of his childhood by attaching himself to a mentor, the evil lawyer Holland Manners. With Holland's help (and an ambition that rivals Macbeth's) Lindsey has scored himself a high-profile job, a nice apartment and a place in the world that seems to suit him perfectly.

Faith's mentor, with whom she hooks up halfway through series 3 of Buffy, is Richard Wilkins, the evil mayor of Sunnydale. He fulfills a parental role in her life, giving her a nice apartment and a place in the world. Sure, she becomes his assassin, but since everyone thinks she's crazy anyway the job seems to suit her perfectly.

Both Faith and Lindsey go through phases of preoccupation with Angel quite regularly, trying to kill him or turn him into Angelus or just generally get on his case. And both of them turn to Mr Vampire-with-a-soul when they think they're ready for redemption.

Love is not something that goes well for them. Faith's attitude, after a long line of failed relationships, is simply 'get some, get gone'. As she falls from grace, sex becomes a weapon for her, which she uses first against Xander by strangling him in bed, and later against Buffy by seducing Riley while the slayers are in each other's bodies. Lindsey falls in love with Darla, and would do anything for her (up to and including arranging for her to become a vampire in order to save her from a terminal illness). In return Darla manipulates him and sleeps with Angel. Yet neither Faith nor Lindsey shows any sign of being truly jaded about love, and both are capable of loyalty and trust, even if it tends to be misplaced.

At this point, Faith is in jail and Lindsey is out on the open road somewhere. Both of them seem to have found a certain measure of closure and redemption. Whether that's the last heard of either of them, only time will tell.