What is a subplot based on: conflict or tension?
I'm attempting to understand how subplots work, and one aspect has me confused. I'm trying to create a process by which I can create subplots. When I create the main plot, I start with conflict - two opposing forces. This doesn't seem to work with subplots.
Let me use the movie Thor as an example. The main plot is about Thor learning his lesson and regaining his power. There is a subplot about Thor's growing romance with Jane. This subplot has no conflict - the romance simply happens. There are no opposing forces.
Another example is Harry Potter. The main plot is about defeating Voldemort, but there are countless subplots. Subplots like Harry asking Cho out in the fifth novel are certainly filled with tension (from Harry's PoV), but I can't say they have conflict. There are no opposing forces.
Then again, there are many subplots which do have opposing forces, and seem to be based on that conflict rather than on tension. So what is a subplot based on: conflict or tension? Or is there a third option I have not considered?
Please note that this question is not about the definition of conflict. If you disagree with my definition of conflict, I welcome your opinions; but please make sure the answers are about what a subplot is based on.