Why is Bangel the ONLY relationship, you ask. Allow me to enlighten you. Okay, here goes. The most simple answer to this is...well, the most simple answer to this: Because Joss wrote it that way. He created two people who were both the exact opposite and so similar that it was almost frightening. First the opposites. He is tall, she is short. and so on: male/female (that's a given, still...) dark/light. slender/muscular, fashionable dresser/could give a s**t less, as long as it's black. Everyone knows opposites attract, it's a law of nature.
The similarities: Each is unique in his/her species, she the only human Slayer, he the only ensouled vampire. Both fight to defend mankind from the things that go bump in the night. He is immortal, while she, while not in herself immortal, is the latest personification of an entity ascending from the very dawn of time.
But so much for clinical details. Their souls touch, entwine, merge, join together in a ballet if love and devotion so pure that it actually needs no consumation, in fact actually is damaged and almost destroyed by the act of physical coupling. She loves him so much that his death literally destroys her world, causing her to flee the comforts of family, friends, and home to grieve his loss. his love for her is so great that he willingly sacrifices the gigt of mortality in order to protect and guard her from harm.
Buffy and Angel are, to use an old cliche, soul mates, bonded together in the fires of battle and the passion of a common purpose. They are unique: mortal enemies by species and star struck lovers by a force neither can deny. The personification of the tragic yet inspiring love, each sacrificing all for the other, neither asking more than to bask in the other's presence. Light and dark joined together in a bond stronger than all the forces of darkness, forever united, always sharing a love beyond passion, and a devotion beyond simple love and duty. Buffy the Vampire Slayer, and Angel, the Scourge of Europe. fused together in a love to rival the passion of the very gods on Mount Olympus. Two champions. Two warriors . Two souls joined tofether in an eternal bond. A love ordained to be.That is Bangel...
(Snort...cough...groan...) "Where am.. what have I...it can't be...I couldn't have...AHHHHHHHHH! Curse you, Willow! A thousand curses and a million more to follow. I'M SORRY, SPIKE, I DIDN'T MEAN IT! PLEASE FORGIVE ME! PLEASE FORGIVE...(sob...choke...groan...)
There you go, darlin'. Let's see ya top that.