I came across an expandable back scratcher recently and it jumped out to me as a great inter-dependent metaphor: You can scratch your own back with it, or you can scratch each others' backs with it. In other words, you can do it yourself, or you can ask or allow your partner to do it for you. I also like the expandability of the back scratcher; it is a great reminder to be flexible and expand your skills for a happy and healthy relationship.
It is not surprising that you wonder how inter-dependence works. I see a lot of independent individuals in a couple. For all manner of reasons, s/he grew up knowing s/he had to count on him/herself. I hear phrases such as:
"I don't need him/her for anything."
"I have to do it myself [or it won't be done right -- or at all."
"My mother/father was at work/drinking/unavailable."
"I learned early on that I can't count on anyone."
Therefore, growing up to be independent makes complete sense; it is a coping strategy you needed. It is a tremendous strength, and a trait that has made you as successful as you are. It has made you resilient. You need this trait. Even today.
If you think about it, every one of your strengths is also your weakness. Being independent is fantastic. On the flip side, it may show, say or feel to your partner that s/he is not needed and/or valued. And being needed is a core need for most people. Remember there is a difference between being needy and needing your partner.
So next time you're itchy, think about your opportunity to build inter-dependence.