This relationship might get serious, but that may or may not be a good thing in the long run. In another lifetime, you two were romantically involved, but it was a heavy, dark affair. One of you betrayed the other, and as a result, some bitter feelings still dwell in your subconscious mind and will come into play in this life if you become romantically involved again.
You'll have a hard time showing each other affection, because of those old wounds that have yet to heal within your souls. At base, you're still afraid of being hurt again; you're afraid that your lover will break your heart and then leave you behind to pick up the pieces. But you may not realize that they're afraid of the very same thing, and clinging grimly to each other, creating a sense of obligation rather than genuine affection and desire, is no way to build a healthy connection. If you act on instinct, you'll attempt to control your lover through withholding affection and being overly serious about your duties to the relationship instead of creating a fun, light, amorous connection. But a wealth of love awaits you both if you can look within yourself at the fears and insecurities that seethe there, and make peace with them once and for all. You still might get hurt, in this relationship or in a different one; the path of the heart is never an easy one. But it isn't supposed to be easy. You're supposed to make yourself vulnerable, to open yourself up to love. Any pain you might experience will only help you to learn and to grow. Don't close yourself off to this experience, and don't restrict the amount of affection and tenderness that you give your lover. They deserve your passion, your best treatment, not a limited and controlled version of you.