Oh man, what wouldn't I do for my special someone (if I had one)?
In all honesty though, there isn't a lot I wouldn't do. If she needs a shoulder to cry on and someone to be there for her, I'll be the first in line. If she wants someone to go shopping with, I'll go shopping (and probably max out my credit). If she wants to go cosplaying at a con, sure I'll do it with her. I'd even give organs or even my life for her if the situation arises.
They're your special someone/somepony because they are special to you. They are the one-of-a-kind person you've imagined and dreamed of. They're the one person you couldn't live without, even though there are times you can't live with them. You'd do everything for each other, and you probably do everything you can for each other. That, coupled with true romance, chemistry, and some fun, is what true love and relationships is really all about.