Responsibility is a key ingredient of this relationship. You're both driven by a sense that you have some important work to do together. What this work is, is up to you -- it could be of a personal nature, such as building a good, solid relationship that leads -- or has led -- to a long-term commitment and establishing a family; or it could be a business venture that requires both of your careful forethought, planning and hard work.
But you work on a project together, and your partner encourages you to work harder and better than you would have on your own. This aspect lends a grounding effect to your relationship. There is a sober feeling to your bond, and though this doesn't have to be a restrictive influence, you might feel dissatisfied under its influence. For a business relationship, sober energy, focus and hard work are fine, even desirable. But in a romantic relationship, you want a little bit of lightheartedness, of freedom and spontaneity, even foolishness. You don't want everything always to have a purpose or a goal; sometimes you just want to lie back with your lover and enjoy the bliss of the moment. This is probably hard for them to do with you. They just naturally focus on the more serious side of life, and their responsibilities and commitments take center stage. They're likely to be in it for the security above all else, which can make your combined social life seem superfluous or stilted to you.