You're out of sync on a day-to-day, emotional level. One of you is usually in a good mood while the other's always gloomy, and that isn't any fun for either of you, because neither of you gets the mirroring and understanding that you need for your feelings. Your daily routines are also in conflict, and you notice this more and more the longer you're involved.
You have different interests; one of you wants to take a walk outside while the other wants a nap, or one wants to see a movie when the other wants to go bowling. Your energy levels are really different; you are much more emotionally buoyant, able to take on the stresses of the day without much complaint, while your lover is more sensitive. You each have to step outside your comfort zone to accommodate the other, and that irritates you both. Your feelings and emotional reactions are very much out of step with your partner's, and you push each other's buttons without meaning to or even realizing you're doing so. As time goes by, you only get more touchy with each other, which could lead to petty challenges and arguments. What is going on, at base, is nothing more complex than two different, unique people who don't quite understand each other, taking the differences between them as a threat. You both have to adapt to the other's rhythms, and be sure to communicate and listen as well as you can.