Angst! Such an important part of sci-fi and such. If there was any justice, these romances would last for eternity, but because one or two tragic reasons, they cannot. We reach for our hankies as we watch these lovers break apart and come together again and again. We know it will never work, but we dream that it will. And then we write and draw those happy endings.
1. Buffy and Angel, Buffy the Vampire Slayer
If you looked up Star-Crossed Genre Couples in a dictionary, Buffy and Angel's brooding screen-capture would be there. A vampire slayer in love with a vampire: How Shakespearean!
Then throw in Angel's soul, making him a good guy, and you have an archetype of which Jung would be proud. They could never be together, because if they were, Angel would become evil again. The one time they made the beast with two backs, Angel went on a killing spree and Buffy shoved a sword through him and sent him to hell. This is the genre couple to which all star crossed lovers are compared.
2. Reese and Sarah, The Terminator
Reese was sent from the future to protect Sarah by her son Connor. Reese pretty much fell in love with Sarah from the moment he saw the Polaroid Connor gave him. Even with the 80s headband she sported.
Sara only went with him because she wanted to live. She fell for his futuristic hunkiness, despite the crazy story he told. The two had one night of sweet, sweet love, thereby conceiving John. Predestination paradox, where would sci-fi be without you?
Then the Terminator showed up and killed Reese. Then Sarah killed the Terminator, because he robbed her of young Michael Biehn.
3. Starman and Jenny Hayden, Starman
Green means go. Red means stop. Yellow means go very, very fast."
Starman was a visitor from another world who took on the face of Jenny Hayden's dead husband in order to learn how Earthlings live. In doing so, he made Jenny relive the loss of her husband, while making her fall in love with him at the same time.
Like her first one with her pre-dead husband, this relationship was doomed to end, because Starman went home.
When he came back for the TV series looking like Robert Hays from Airplane instead of a beardless Dude from Big Lebowski, Jenny was missing and left their son behind, guaranteeing the Weltschmerz would continue.
4. Daniel Jackson and Sha're, Stargate movie and Stargate SG-1
They changed her name when they made the TV show, but essentially she was the same simple country girl Daniel Jackson initially rejected on the big screen. Despite this rejection, she stayed with him and taught him to speak Abydonian. Eventually Daniel fell in love with her and stayed behind when Jack and the others returned to Earth.
She was also the same simple country girl who was taken by Apophis to be the host for Amaunet and the mother of their child. And she was the same girl who tried to kill Daniel, but in the end sacrificed herself so he could live. Daniel, ya should have gone with your first instincts.
5. Gambit and Rogue, X-Men
He was the bad boy from the Bayou who eventually turned good. She was the bad southern belle from Mississippi who turns to the X-Men for help dealing with powers she absorbed. It was natural they should fall in love. Unfortunately, they could never consummate their love because Rogue would literally suck the life out of Gambit.
And thus many an angsty scene was born.