Relationships are like muscles, if we exercise them, they will grow (even if we don’t see that growth).
Marriage takes work, and will not naturally grow on it’s own. In fact, left alone, a marriage will shrivel up and die. It takes consistent time and energy much like your muscles.
If you were to sit all day every day for a year, you would notice a significant amount of atrophy in your body. Your inability to function after that year of sitting would likely take you a more painful and greater amount of recovery to return to your previous abilities (if ever at all). Once you have lost muscle mass, it is very difficult to get it back.
Your months and years of dating and courtship are very much like a daily trip to the gym. You exercised the muscles of the relationship that allowed it to grow. Though for a lot of us, when we married, we stopped going to the gym (literally and metaphorically!).
Continue your visits to the gym. Read books together, attend marriage workshops, go on dates, spend quality time together, give each other gifts, surprise each other with good things, and take trips. Do all of these things regularly and your marriage will thrive.