I have two at the moment, might be three or more if I can get another round of stories out.

Human (EqG look but with Spike actually being human)- Spike lost his real parents when he was around eight years old and lived in a temporary orphanage. After a few months, he was adopted by Twilight and they moved to Ponyville where he began to attend school in Miss Cheerilee's class along with Sweetie Belle, Apple Bloom, and Scootaloo. Diamond and Silver began to make fun of him until Sweetie Belle stood up for him, leading to them becoming fast friends to Best Friends. During the next so many years, Spike and Sweetie began to harbor feelings for each other and eventually they confessed and began going out. (For more, well, the stories aren't complete just yet)

Pony (movies)- Still working on that one.

What's yours?

4235447 My headcanon: They made a connection through Rarity. Both presumably spend an ample amount of time at the boutique, there's a slight chance the two have even helped her on a design or two, and it is more than likely that they have become, at the very least, friends, maybe close friends as we can all assume so with the season 2 finale. My headcanon goes no farther than this.

A small problem with the ship is that the two have barely interacted outside of their respective group. The CMC are basically intertwined, Spike is joined to Twilight or Rarity at the hip. It's hard to have just a solo Apple Bloom, Sweetie Belle, and Scootaloo Episode without, at the very least, some mention of the other 2 members. It's difficult to have a Spike episode without him A) Being completely and utterly stupid(Spike at Your Service). B) Having Something to do with Rarity(Inspiration Manifestation(is that the title?)). or C) Having him featured in personal drama (That greed episode/Dragon Quest/ Equestria Games).

So outside of the wedding dance, it's difficult to justify why this works, which makes building a headcanon difficult. Maybe they could bond over music as Spike has been shown to play the piano and Sweetie Belle can sing, I don't know, though logically they could have a duet, maybe Spike could even help her learn to like her singing....

4235657 That is true, but seasons three and four have shown Spike interacting with the CMC more and more. Here's hoping season five has more of that. At the very least these two being together is more plausible than Sparity.

My headcanon involves Spike finally realizing that Rarity will never see him as more than just a friend. He goes to Rarity's to ask her out on a date, only for Sweetie Belle to tell him that she's busy. Spike decides to take Sweetie out for a date instead and the two of them really hit it off.

I'm don't really have one strong headcannon, although I would lean towards Spike spending lots of time at Rarity's boutique and spending more time with Sweetie Belle, over time Sweetie begins to develop a crush on him. After a while Spike begins to feel the same way and eventually one way or another they admit their feelings. There are many ways this idea can be excited so I keep my mind open to ideas.

My head canon is the same as everybody else. Sweetie Devolves on Spike.