Love is a big word. Sure, most people might think of the love between two people, the kind of innate emotion that leads to marriage, children, and a house in suburban Stormwind.
But the first kind of love story that I always find convincing is that between siblings. Malfurion and Illidan themselves are a fascinating sibling love story, because for all of their arguments, struggles, and wars -- they were brothers. They (presumably) loved one another, and that familial relationship built an innate stress and tension into their interaction.
Your sibling relationship isn't limited to rivalry, however. Consider a story about a sister who brought her brother to Northrend to war against the Lich King . . . and is now horrified to discover her brother has become a Death Knight. He's risen again, and is still fighting the "good" fight. But he's kind of undead, and a palpable reminder of how she led him to this dark place.
My favorite familial love story was one of my first World of Warcraft roleplay experiences. On a whim, I randomly decided that my paladin was a father, spurred to go fight the hated Blood Elves because they'd killed his son. It wasn't a simple war kill: instead, the son had been another paladin, killed when the Blood Elves attacked the temple in Stratholme.
Okay, so familial love is a fairly easy story to write. But it's admittedly not a traditional "love story." As much as I might wish it so, no one thinks of the love between father and son when they hear the words "love story."
There's probably about a dozen different romantic love stories. They meet-and-swoon, the meet-and-hate-then-swoon, and the meet-and-starcrossed are probably three of the most common. I see a lot of hate towards the meet-and-swoon, since it's probably the biggest cliché of them all. But, still, it's valid, and it's a viable choice for folks who just want to get into the nitty gritty of their love story.
The meet-and-hate is a lot of fun, though. It helps if both players are prepared for the roleplay, already girded up and set to really tear into one another. Basically, two characters meet, and start off their mutual life despising the other. Maybe one character has a long-term resentment of the other character's race – after all, it's not like all of the Horde and Alliance races are filled with fluffy bunny love for one another. (Or do we really think the Tauren and Undead are BFFs?) Once you've established a good hatred for one another, write some circumstances that start forcing the two character together.
Maybe the two characters get pinned down in a foxhole, dodging Scourge mortars and attacks. They're stuck in that hole for days or weeks, slowly forming a begrudging respect for one another. Or, perhaps, the two characters are sent on some kind of secret mission for their faction. And through peril and adventure, they slowly come to love.
The meet-and-starcrossed story is a little more difficult to make convincing, because it relies so heavily on a certain amount of deus ex machina. Think of Romeo and Juliet (for which I've named this roleplay method, obviously). It takes a lot of setting and theme to make that story work out. Two noble houses hating one another, and a certain amoutn of fate and luck for Romeo to score a ticket to Juliet's house.
These options are much more commmon in World of Warcraft, however, because of the elegant metagame themes. Characters are constantly be tied to one another through question, instances, and overall war themes. You can use similar techniques to hook two characters up as you would if you were writing a meet-and-hate, except your two characters don't innately hate one another.
These are just three ways to help get your characters' love stories off the ground. They shouldn't describe everything about your ongoing storyline. Most roleplayers tend to prefer organic stories; which is to say they slowly form and build over time, according to the in-game actions you take.
But, it sometimes helps to have a general idea of "theme" for where your love story is going. For example, you might decide you're not looking to tie all of your roleplay down to obligation to one other person. That makes an awful lot of sense if you're a casual, intermittent player. (After all, if you're married and living with someone, that requires a darn lot of communication between the two players about what your two characters are doing.)
So given that fundament, maybe you don't want this relationship to work out. Then you've got to chart out how the love story ends. Does it go down in flames when one character commits adultery? Do they casually, slowly drift apart? (It's not me, it's you.) Or, even worse, does tragedy strike, and one character dies. (And again, bring one character back as a Death Knight for that added angst!)
Love stories have a lot of meat to them, because it plays with such fundamental human emotions and needs. Exploring these options can significantly enhance your roleplay, althought it will almost definitely take a hefty level of maturity to navigate those murky waters. I'd really love to hear some about how your experiences with these stories have been – drop by the comments, and let me know what your best relationships have been like in WoW.