You ran away from home out of frustration, only to fall into a hole in Mt. Ebott. Luckily, you were still alive. How is that even possible? Either way, you're glad that you were still alive and well. You decided that it would be your goal to go home -- to go back to the surface. Along the way of your journey, you met good people, and a couple of bad people. But, you also met someone who you'd never thought you would end up being romantically involved with. And that "someone" had a name.
And his name was Sans.
Sans the skeleton.