For me it's natural. When I was young I liked to tie myself up (when you play alone there is only one person to tie and get tied). I was five.
When I played cops and robbers with my friends I liked to tie other people, though sometimes I was jealous of them. Before I ever knew what dominance or submission was I liked to do both, or either.
I love the feel or rope on my skin and I also love how other people look when I tie them up. Life would be terribly uninteresting if I was forced to go one way or another.
Ultimately, I think, the biggest draw for me is being with the person I care about most and sharing a good time together. It fun to think about her tied down and unable to avoid my attentions. It's more fun to picture her being denied and getting frustrated, only to end in a tremendous explosion, when I let her of course.
And I like her to do the same for me.