Working together is a wonderful part of being in a relationship, but you tend to overestimate your abilities and your resources as a couple, and your lover probably gets swept up in your grand vision. You talk them up, whip them into an excited frenzy -- and then you both find out, sometimes too late, that you can't really accomplish what you thought you could. You're competitive together, though, and you want to take on big projects -- even when it's not the best idea to do so.
This comes from a good place, in the sense that it results from a feeling of positivity and expansiveness that grows whenever you're around each other. You, especially, think that you can tackle anything as a team; and then you find out that that's not exactly true. Your beliefs and ambitions might not match up as well as you think they do in the beginning of your relationship, and after a while, it becomes apparent that you're working at cross-purposes. It could come as a surprise when you think you've been working in tandem but find out that in fact, your efforts have been going directly against each other, in effect canceling each other out! Sexually, this could be a problem as well; you might be out of balance with each other in the bedroom, or you might go overboard in your sexual expression, making your partner feel turned off as a result. As usual, compromise and making an effort to really listen to each other helps greatly.